Wow. That took a whole 2 minutes to figure out.
Oscar Peterson -> Paul DeMarky -> Itsvan Thoman -> Franz Liszt.
You can absolutely say that is a true fact. There is no dispute anywhere in that lineage.
Thoman was APPOINTED BY Liszt to teach at the Royal Hungarian Music Academy, where he also mentored and taught Bela Bartok.
DeMarky was at McGill University in Canada. I couldn’t find too much on DeMarky as he’s not a well known pianist, but to even get a position teaching in a college like that, you would need credentials which would probably include who you studied under.
So the Thoman - Liszt relationships is rock solid certain. And we have no reason not simply take Oscar at his word that he studied with DeMarky. I would be VERY surprised if the DeMarky - Thoman relationship turned out to be untrue.