That was an early hypothesis I considered, but it doesn't fit the data. That would show up as a larger tabs/spaces salary gap for big companies, when in fact the gap is similar across all company sizes:

https://twitter.com/drob/status/875493967865008129

Perhaps it could play a role as a more indirect effect: people leaving Google/Microsoft/Apple/etc, starting new companies or joining senior positions at others, and spreading this particular practice.