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وكأن القائلين بأن ظاهر الحديث أنه رضع من ثديها لم يقفوا على شيء . فقد روى ابن سعد عن الواقدي عن محمد بن عبد الله بن أخي الزهري عن أبيه قال : كانت سهلة تحلب في إناء قدر رضعته ، فيشربه سالم في كل يوم ، حتى مضت خمسة أيام ، فكان بعد ذلك يدخل عليها وهي حاسرة ، رخصة من رسول الله صلى الله عليه وسلم لسهلة
...As if those who toke the hadith literally and said she gave him his breast have never hear/read anything. As it was narrated by ibn Sa'ad from al_Waqidi from Muhammad ibn 'Abdullah the son of az-Zuhri's brother from his father who said: Sahlah was milking a quantity for his breast feeding (of her milk) in a vessel. Salim drunk this each day until fuive days have passed. Afterwards he (could) enter (her house) while she was bareheaded (unveiled), as a permission from the Messenger of Allah Allah's prayers and blessings be upon him to Shalah.

Scholars interpret it as a respect, as he asked al-Qassim again to get his authorization to narrate the hadith. And this goes ahead with an-Nasa-i's narration of the hadith:

وكأن القائلين بأن ظاهر الحديث أنه رضع من ثديها لم يقفوا على شيء . فقد روى ابن سعد عن الواقدي عن محمد بن عبد الله بن أخي الزهري عن أبيه قال : كانت سهلة تحلب في إناء قدر رضعته ، فيشربه سالم في كل يوم ، حتى مضت خمسة أيام ، فكان بعد ذلك يدخل عليها وهي حاسرة ، رخصة من رسول الله صلى الله عليه وسلم لسهلة
_{My own translation take it with all necessary care.}
...As if those who toke the hadith literally and said she gave him his breast have never hear/read anything. As it was narrated by ibn Sa'ad from al_Waqidi from Muhammad ibn 'Abdullah the son of az-Zuhri's brother from his father who said: Sahlah was milking a quantity for his breast feeding (of her milk) in a vessel. Salim drunk this each day until fuive days have passed. Afterwards he (could) enter (her house) while she was bareheaded (unveiled), as a permission from the Messenger of Allah Allah's prayers and blessings be upon him to Shalah.

Scholars interpret it as a respect or maybe "fearful respect", as he asked al-Qassim again to get his authorization to narrate the hadith, because on one side the involved people were alive and on the other side the majority of sahabah and the mothers of believers rejected the idea of breast feeding an adult to gain mahram relationship (as is stated in the reference proposed by @UmH). And this goes ahead with an-Nasa-i's narration of the hadith:

So this narration makes clear that the other wives of the prophet () had a different opinion on the matter. Az-Zurqani in his commentary on al-Muwatta' added a narration actually explaining how Sahla gave milk to Salim:

So this narration makes clear that the other wives of the prophet () had a different opinion on the matter. And this is the view of the majority too so this could be another reason why ibn abi Mulaikiah didn't narrate this.

Az-Zurqani in his commentary on al-Muwatta' added a narration actually explaining how Sahla gave milk to Salim:

I actually don't know anybody else than ibn Hazm who said that suckling happened or was necessary for the case of breast feading an adult.

This is well known as procedure which has been mostly narrated on the authority of 'Aishah. See also Sunan ibn Majah & suckling. This hadith actually appears almost all hadith collections except with Sahih al-Bukhari, here a quote from a longer version from al-Muwatta':

Scholars interpret it as a respect, as he asked al-Qassim again to get his authorization to narrate the hadith.

This is well known as procedure which has been mostly narrated on the authority of 'Aishah. See also Sunan ibn Majah & suckling.

I actually don't know anybody else than ibn Hazm who said that suckling happened or was necessary for the case of breast feeding an adult. Only al-Albani -among contemporary scholars- agreed to this. And scholars already discussed this earlier.

This hadith actually appears almost all hadith collections except with Sahih al-Bukhari, here a quote from a longer version from al-Muwatta':

Scholars interpret it as a respect, as he asked al-Qassim again to get his authorization to narrate the hadith. And this goes ahead with an-Nasa-i's narration of the hadith:

It was narrated that 'Aishah said:
"Sahlah came to the Messenger of Allah and said: 'O Messenger of Allah, Salim enters upon us and he understands what men understand, and knows what men know.' He said: 'Breast-feed him, and you will become unlawful to him thereby.'
(Ibn Abi Mulaikah, one of the narrators said:) For a year I did not narrate this, then I met Al-Qasim and he said: 'Narrate it and do not worry about it.'" (Sunan an-Nasa-i)

See also this Article in Arabic showing more details and discussing what somebody concluded from al-Albani's statement.