Dropbox was an unusual case in that Drew built the product and got into YC before getting Arash to join. I guess maybe a lesson is that you can invent and code up something like the early dropbox on your own but it's useful to have a cofounder when employing a bunch of people and building a company?

It may have started equal at founding and changed over subsequent rounds. I know of a case where 4 founders started equal and differed by order of magnitude by the end.

How did this happen?

Many will sell some of their stock during a funding round to the new investor to take some money out and blow it on lambos.

Come to think of it - I don't many successful startups where the split is exactly equal prior to the IPO.

You'll find in the twitter replies that most people don't agree with his anecdata.

How is that not counterevidence given the significant split between the two?

I think that may be his point (that PG views that as an appropriate split for one or more reasons).