Having taught a fair share of liberal arts majors in intro CS courses, I am absolutely certain that there is essentially no difference in skill between people who choose to major in CS and people who choose to major in the liberal arts.
No. Most liberal arts majors either doubled in a "harder" field, or minored in Education/Business/etc. as a back-up option. The Edu and Business major and minor required a CS course. So most of the liberal arts students ended up either minoring in a STEM field or else taking CS1.
If anything I would assume the opposite of a selection effect.
I, personally, double majored in philosophy and computer science. I think the upper level CS classes would have washed out most of the philosophers, but I agree that the intro classes were fairly accessible.
Liberal arts majors (especially art majors) probably work harder than CS majors in many cases. I still think they would wash out of a STEM major. Not for lack of intelligence, but just because they're not wired that way. They'd be miserable in higher level classes.
Lots of CS majors also aren't "wired that way" (lots of gamers who've never been good at math or science but are "good with computers", kids who hear the jobs pay well or are enamored with prospect of striking gold, etc.). I've tutored a lot of juniors who have trouble in an algorithms course because for loops are still difficult for them.
I don't think these people are more well-represented at the beginning of Liberal Arts programs than at the beginning of CS programs.
This may be less true about some of the less "hot" STEM fields.
Having taught a fair share of liberal arts majors in intro CS courses, I am absolutely certain that there is essentially no difference in skill between people who choose to major in CS and people who choose to major in the liberal arts.