MC might be able to fully remember this time as well as opposed to vague trance memories that everyone gets.
Do we know when that falling out was? Unless Cole was getting experimented on as an infant

the fallout would have had to been less than 10 or so years making Cole a child (but cognizant) during the experiments.The only way I can see both of them having done it is if Tanos was "working" at the lab and xanthe was "helping" at the lab despite having a (comparable or better) lab at hero (or maybe xanthe only got involved after MC joined but before cole)
Edit: Or tanos was working at some third as of now unknown place between the time of Ella picture and just a bit after her death when he joins SIN