First, interns at big-name tech companies are highly overpaid for the work the company gets now; it's seen more as a hiring cost than a salary. The company pays ~$50k (~20k is three month's salary; add in housing stipend, taxes, and overhead) per intern in the hopes that a decent proportion of them will be back. Those that are back are solid engineers that are known quantities; false positives should be much more rare than in your general hiring process because you get to see them work in your company for three months before deciding whether to make a full-time offer. Given how hard and expensive engineer recruiting is, this is an ok deal for a lot of companies. But regardless, comparing to intern salary isn't meaningful because few companies actually believe that they are paying for what the intern will produce in their three months when they pay top dollar for elite interns.

Furthermore, while higher salaries are certainly available, those salaries don't seem absurdly low for the stage of company they were (tiny staff, no product released). Most companies at that early of a stage do expect employees to accept a nontrivial cut from their market rate in return for more interesting work, more autonomy, and fairly significant (if high-risk) equity.