One of the salient differences between the 90s tech bubble and our current state of affairs is that in the 90s Yahoo sold to startups justifying a higher valuation for Yahoo justifying more startups... and this largely doesn't happen now.
I haven't worked with Github but I have worked with many companies which are "strikingly similar" to Github. Their median customer is a boring business which sells things to people for money. If the entire Bay Area slid into the sea, most software companies wouldn't notice until their pull requests stopped getting accepted.
While I agree with you that things are different this time, I still suspect that there is a fair bit of growth acceleration coming from startup dollars being recycled back through the startup economy.
Don't look at me. We didn't raise VC either. Nor will we, until I take off this "WWMCD" bracelet.
I can be a very bad sign, under certain conditions, sure. It's not meaningful all by itself, though.
Also, sometimes they have to take a lot of money, as later-stage investors want to control a sizable chunk of the company (in this case, 12.5%).