What Sean left out was -- overfunded for whom? He implied it, but didn't say it: the bigger dogs in the fight (Facebook/Twitter/Google) have to compete for talent like everyone else. The overfunding is mostly a problem for those larger tech companies, not the little guys.
I suspect he means that investors are overvaluing small startups (relative to their chances of becoming successful). The consequence he worries about is that having too much funding available makes the opportunity too appealing to the best engineers, making it harder for ventures that will succeed to keep/attract talent.
He's arguing against capitalism, and not making a very good one at that. While trying to sell this as an industry-wide issue, I suspect his real concerns are more immediate -- for his own ventures.