2. I don't necessarily agree with the premise, he's not looking for a greenfields project, he's looking to use the toolset he was sold on during the interview process. There's plenty of established companies that have moved from xyz -> React in the past year or so.
This seems a classic case of being either deceptive or self-deluded from the interviewing side, with classic results. Its foolish, it rarely results in good things, and it's a huge waste of time for everyone when interviewers are not straightforward with candidates. Don't do it, and I would not feel bad for a second leaving a place that pulled that.
According to OP this company has been around for 10+ years. It'd be a miracle if they didn't have legacy code.
After working in this business I don't think I'll ever be able to return to a "normal" type of business, although stress can be quite high and often there is overtime to meet a deadline.