BG3 was never marketed as a strict DnD 5e game. It was marketed as an RPG "based" on DnD 5e. According to Larian it's meant to be a hybrid of RPG and 5e. So, if someone asks for an option, it shouldn't be a big deal.
Sorry, but this needs a separate answer: no, this is not true. They did promise a faithful translation, with some minor tweaks to certain rules (they gave example) for better implementation RAI (rules as intended) when RAW (rule as written) does not work. They NEVER talked about any hybrid system, or a homebrew system. Whenever rules come to question during numerous interviews, it was always "5e and nothing more".
DOS system remained - as I think (and I can be wrong on it) - for their DOS audience, who knows Larian for DOS games and came to play yet another game in their system. But it clashes with 5e rules on a fundamental level and if you advertise the use of one system and people bought the game specifically to play that very system, you have to stick to that. Otherwise, it's a false advertisement.
