Are you implying that they should be paying much more just because the company can afford to? This doesn't seem like good business sense, especially when they're already paying above market rate (and based on the highest market rate in the world, at that).
My thoughts are that people like Sam and Jeremy have years of open source contributions to Rails that which make their way into Basecamp, are world class programmers and would easily be 500k+ engineers at a FANG company. And they should pay them as such.
At the end of the article, he says: Just remember to go at your speed. Basecamp wasn’t paying these top end salaries when we started. Or even after we’d been around for five years. We do it because it’s possible now. Not because we’re putting ourselves in a bind trying to keep up with Mr Venture and Mrs Capital.
The vast majority of those comp packages are due to stock appreciation, and in the cases where they are not, it's due to competition between FAANG companies - all of which require you to relocate to a super high COL area and go into an office and deal with a questionable corporate environment.
None of this applies to Basecamp, and as much as I'm a fan of Rails I've spent a lot of time in that source and am not of the mind that a top Rails hacker is doing work nearly as difficult as the more sr. staff at a FAANG.