Well even before we discover that, considering they made the demo for PS5 so it would be strange if they didn't praise Sony for the system as well.
Would be good if Gabe gave his reason, even if it simple "the specs are higher, GP, play anywhere, etc" even though that is probably what he thinks even without saying.
The issue with Tim/Epic wasn't so much that they praised PS5, just praising PS5 is fine, the problem was that they did like 5 or 6 different interviews praising it, each basically saying the exact same things about PS5, and the only time they mentioned Xbox Series X was to say "oh yeah, it could also run that demo" after they got called out by 3rd party tech websites and such for implying that only PS5 could handle that Unreal 5 tech demo. As a 3rd party publisher, it always seemed like they were lacking objectivity in this console war (especially considering the fact that far more MS studios use Unreal engine than Sony studios), and then when the news broke that Sony had acquired a portion of Epic Games, it became pretty clear why they didn't seem objective.
Of course, Gabe Newell isn't really all that objective either. He used to work for Microsoft (though I don't think they parted on the best of terms), Xbox releases alot of their games on Steam now and offers crossplay with Steam on some games I think. Of course, there was a brief partnership in gen 7 between Valve and Sony with some kind of plans to release steam on PS3, but I believe that deal fell through with Steam never actually releasing on PS3. And Gabe Newell also criticized Windows 10 Store quite alot when Microsoft first released it as a competitor to Steam, so I guess he could be objective in this case when he says he prefers Xbox Series.