I don't think there's some sort of "line" that Delve crossed and Corgi has not, more likely the financial upside for YC wrt Corgi is much, much bigger. They're not going to mess with that even if the screw over a few of their other startups. YC absolutely loves to tout this family dynamic across all their companies and alumni, but there is no way the relationship between a few dozen startups looks anything like that between literally thousands of companies. It's a scaled up tech-hype startup machine now, and has attracted the same MBA-types they supposedly defeated 20+ years ago.
The last sentence is false: More than ever, YC is most seeking highly technical founders.
It's also false that YC will overlook IP theft for the sake of financial upside. One of the most fundamental principles that YC drums into startups is that they shouldn't breach software licenses. This comes from personal, painful experience for pg in the Viaweb days; a dispute over IP nearly killed their acquisition by Yahoo (or a major funding round soon before that). YC counsels startups to be super-prudent and upstanding about software licensing. Sure, sometimes startups will be cavalier about it and cross a line, in which case YC tells them to fix it immediately.