First of all, there are different explanations of what this verse means. Some people like Tabari (following the opinions of Ikrimah and Ibn Zaid) understood this verse to refer to the rain and plants.
Meaning, the sky and land were uniform, and Allah sent down rain from the sky and brought out plants from the earth. In this interpretation, there is no problem because both rain and plant growth is seen by the disbelievers.
However, how would the verse work if it meant the common explanation today that the sky and earth were one mass before they were split apart?
Razi mentions in his tafsir:
If someone says: "Either ⟪Do the disbelievers not see⟫ refers to sight or knowledge. As for sight, it is problematic. Firstly, people definitely did not see it, and secondly, Allah says ⟪I did not make them witness to the creation of the heavens and the earth⟫ (18:51). As for knowledge, it is also problematic. Objects in essence can be together or separated, so saying they were first together then separated is only known by being told. The debate is with disbelievers that do not accept the message, so how can this sort of thing be used as evidence?
He answers this question in a couple different ways, and I will mention summarized points:
It seems to me that the second response is stronger. It is known in many mythologies that they have a story of creation that involves there being one thing first before it was separated into the universe. It is possible the Arabs had something similar.
This is plausibly because of remnants of the knowledge from past prophets. It can also be that they borrowed from Jewish or Christian sources.
However, there is another response to this question I did not see. Namely, that the construction "Don't the disbelievers see" is only being used rhetorically. Perhaps, it does not mean they literally saw or know this. It may just be a way of Allah informing them of this fact.
This may fall into the first response Razi mentions. Allah informs them of this and uses it as evidence of order in the creation because the revelation of Muhammad (SAW) is proven.
And Allah knows best.