The $155k salary might be due to the transparency, but could it also be due to a simple inability to pay developers $300k+? That's well over double the CEO's salary. I think it's clear they simply don't have the revenue and cash reserves to pay developers that kind of money. I suspect you'll find this to be true at a lot of smaller startups that rely on actually making money instead of fundraising.
Also, Buffer is entirely remote. I bet many people are willing to take a big pay cut if they don't have to commute to an office building every day (and of course, other people like office life -- but remote positions are rare, so it's normal that they're more "expensive" for employees who really want them).