In this translation of a hadith attached to Imam Malik’s commentary:
"if someone changes his deen, strike his neck!" refers to those who leave Islam for other than it - like the heretics and their like, about whom it is known. They are killed without being called to tawba because their tawba is not recognised. They were hiding their kufr and publishing their Islam, so I do not think that one calls such people to tawba, and one does not accept their word. As for the one who goes out of Islam to something else and divulges it, one calls him to tawba. If he does not turn in tawba, he is killed [ Translation of the Muwatta Malik Book 36 Hadith 15 ]
I am having trouble understanding what is meant by
like the heretics and their like, about whom it is known.
Please explain to me in other words, what kind of Apostate Imam Malik is referring to when he says
their tawba is not recognised