The context of the quote was "I've been invited to poly weddings, but I've never been invited to a poly 5-year anniversary". I assume that it referred to the anniversary of said wedding. Thing is, though, I've never been invited to a 5 year anniversary ever. Mono, poly, whatever. People tend to invite you to their wedding and have their anniversaries on their own. So that seems like a weird thing to say.
We all know people who've been in long-term poly relationships (be it only on these forums) so we know it happens, but I can see how, since long-term mono relationships are rare already, and long-term poly seems to mean (according to the people commenting, at least) time spent in a relationship with ALL partners, well, that's going to be rarer, just because there are more variables. But yeah, it happens. And someone who is poly all their life and has several 10-20 year long relationships isn't suddenly non-committed just because they only overlapped for 5 of them, or something.