Is there any particular reason you say "probably" here?
Not arguing, as it being possibly true mentioned as part of why Basil I may have disliked Leo so intensely in everything I've read, just wondering if you've run into anything especially conclusive.
DNA testing would be nice, but that means we need the bodies to be tested.
Here's a clip from Wikipedia so everyone knows what we are talking about:
Michael III's marriage with
Eudokia Dekapolitissa was childless, but the emperor did not want to risk a scandal by attempting to marry his mistress
Eudokia Ingerina, daughter of the
Varangian (Danish) imperial guard Inger. The solution he chose was to have Ingerina marry his favorite courtier and chamberlain
Basil the Macedonian. While Michael carried out his relationship with Ingerina, Basil was kept satisfied with the emperor's sister Thekla, whom her brother retrieved from a monastery.
I used the weasel word 'probably' because I haven't read the original sources; I'm taking what modern secondary sources have to say about this on trust. As you've already said, in the absence of genetic testing there's no way to know for certain that Basil never slept with his wife in Michael's lifetime, but Basil's attitude makes more sense if Leo was Michael's son.
I don't even think it's possible to know whether Leo looked more like Basil or Michael. Byzantine coin portraits are poor and the mosaic portraits aren't distinctive enough to draw any conclusions.