He just saw it as another opportunity to re-live his fantasy of a perfect love story.
In Lu Feng's case, he doesn't really give a damn about keeping Yichen or their relationship a secret.
That's what makes him scary dangerous, I think. The man is acting like he doesn't have anything of value to lose except for Yichen. Threaten to sever his ties with Yichen and he'll do whatever it takes to get it back.
He is already aware that Lu Feng will do anything to get what he wants. Too bad for YIchen, IT IS HIM that Lu Feng wants 🙃
(Again, the guy was obsessed ever since middle school. You'd think he'll let go that easy???) I think a 20-year span is too big, it feels like they wasted a lot of time in life in the novel. Them reuniting at +30 is perfect.