If you can't make a web site work with Safari then you have no business calling yourself a web developer. Not even joking.
I was under the impression the discussion was about developing extensions for Safari, not websites.
I am perfectly capable of making a website work in safari but I am not able to test it because I don't want to spend $xxxx buying a macbook for the purpose of refreshing a web page. Firefox and chrome run on my dev machine and Microsoft gives out a free VM image with edge.
I'd say the majority of websites were never tested on safari because most small dev places can't afford to buy macbooks just to test one browser.
They could use Sauce labs or some similar very affordable solution.
Potentially. Although in my experience safari works 99% the same as chrome so I'd bet a bunch of devs just assume if it works on chrome it will work on safari.