Yes, but the most important fact here has not been shared. Did Sofia participate in an event that ran concurrent to a WPBA event? If so, then her choice to play Ultimate Pool was a big deal. If not, then I understand where everyone is coming from, and the fine seems a bit odd and the policy very restrictive.
I must open with the fact that my knowledge on this matter may not be current. That said, I have some familiarity with and have seen some of the WPBA contracts of yesteryear.
The contracts I've seen only restricted players from playing in other events that ran concurrent to a WPBA event. Actually, that restriction made perfect sense, as when the WPBA sits down with a potential venue sponsor, it seeks a sanctioning fee from that venue sponsor and that venue sponsor, before agreeing to pay the sanctioning fee will, typically require that each WPBA player under contract will either play in the sponsor's event or, at least, not play in any other concurrent event. Without such a pledge, venue sponsors are much harder to line up. Hence, the WPBA player contract helps the WPBA to make business deals.
... and let's make no mistake about it, Sofia Mast is elite in women's pro pool, with multiple top 5 finishes in sanctioned world championships.
About fifteen years ago, Jasmin Ouschan played in the World 10ball, even though it coincided with a WPBA event, and she was fined, because her player contract was breached. Ironically, she won the very next WPBA event in which she played.
WPBA prize funds have skyrocketed in the past few years, but the bottom line is that the players who sign the WPBA contract have negotiating power and should be mindful of what they agree to for the 2027 player contract. The WPBA is now a 50 year old tour and has lasted this long due to generally capable management. I am sure they will all work this out.