The argument you're using here about the game depending on MS APIs and such is simply not an argument against releasing, and you know that perfectly. It is in fact an argument for releasing it open source. There's no point at which it's ideal to release the code - it can always be cleaner, more optimized, saner, and so forth.
Here[1] is an example of someone using that argument and delaying the release by first a decade, and then saying it probably won't happen at all. It's just complete nonsense - if they were afraid to release their code because it was "ugly" or "not modern", again, that's not an argument.
But obviously I have little to say about your last paragraph, except to reiterate what someone else said and asking if the code could be released without those parts :-)
1: https://www.phoronix.com/scan.php?page=news_item&px=Lightwor...