I think you're both a bit wide of the mark.

- Claiming to be discriminated against (come on, really?) and demanding action is childish and ridiculous.
- Claiming that cross-platform development is so hard that's it's ok to mislead Mac customers into purchasing early, only to compound it with inconsistent, vague messaging is equally unacceptable (I know you ended with that, to be fair!)
I purchased the game back in August on Steam because, back then -
as now! - it claimed to be released on Mac. I had the option of waiting for the PS5 release but thought I didn't need to. It was only when I ran the game and was confronted with the "early September" message that I realised I'd made a mistake. But so what? I had no problem waiting and it appeared that the Mac and PS5 releases would coincide so I was neither losing nor gaining anything. Fast forward to today and there is no release date in sight, and I could be playing now on my PS5. Naturally, Steam won't refund my purchase.
This is mostly a criticism of Steam but Larian doesn't come out of this well, either. The game still teases an early September release and, more than a quarter into the month, that's stretching the definition of "early September" somewhat. All we have is a Twitter post to tell us that it's now "later this month" but the fact that they can't share a date tells me that there's a problem - their project management system (whatever that might be) would certainly be suggesting a date unless there was a critical path activity with an unknown duration.
To Steam, sort out your false advertising. I've been a customer of yours for 17 years and I'm really disappointed in your actions. If you were in the UK you'd be toast.
To Larian, please, at least update the in-game message to coincide with the Tweet - if we can't get a straight answer, please at least give us a
consistent, vague one. And please tell Steam to change their messaging!