...a BAD idea for people to go around buying up expired domains (or soon-to-be expired domains) and redirecting them to one site in order to "harvest" the incoming links...
This, IMO, is why Matt in the video artfully avoided mentioning certain aspects of redirection... and it's also what made my answering your earlier question tricky. I didn't want to bring it up either.
Google would certainly view this and similar uses of redirection as manipulation rather than as a natural sign of quality, and it's a technique I wouldn't use to boost a domain I valued. I know that Google has tools that can detect this kind of redirection... but I don't know whether they can apply it on system-wide basis. Redirected links are harder for competitors to spot. I haven't checked out how various link data bases report 301s.
Over the years, I've seen the technique used for various purposes that I'd call unethical, not just spammy... and it is something that Google will treat harshly.
I should add, btw, that buying up new
keyword domains and 301ing them to a main site with expectations that the keywords alone in the redirected domains will boost rankings is up there with meta keywords and the tooth fairy. As soon as a domain is 301ed, the keywords in the domain are gone.
What may be left after such redirects are any anchor text links still in place to the keyword-rich domain that's been redirected. It seems to me though, particularly in situations where you control the sources of inbound links in the first place, that its pointless to go through the charade of putting up a domain only to redirect it. Just put up the spammy link.
The slap from Google is also likely to be a lot harder for hidden redirects than for simple spammy anchor text links to the main site, though scale and frequency of transgression also enters into this.