Oh, ok I see.... Ya, well I guess that's a short term issue but once they kill the Flash access that won't be an issue. WGT won't keep the Flash version running, that's a given.
It's not that Steam is letting anyone into anything, it's whether WGT is building cross platform multi-player access into the new game. Technically they could allow 'browser' players and Mac or PC players to play in the same ALT match for instance, all three games would be accessing the game via WGT servers so no reason they couldn't.
The issue would be as you say, Steam players would be subject to their anti-cheat and browser players would simply play as usual, which would be ridiculous.
To me it's not even an issue, WGT is going to kill Flash access at some point, I think they'll do it before the end of 2020 myself.
The worst thing to happen is that they don't release it on Steam and just keep it as is, keep tweaking and polishing the game, basically following the same M/O as the Flash game. I think they will though for sure, just makes sense, no logical reason not to.