It's an argument, but not a good one. Don't be evil they said...
It is very uncommon these days for actively maintained websites to not support Safari/iOS users
Well I am not one of those people. Back in the late '00s I was testing in IE6, 7 and maybe 8 as well as Chrome, Firefox, Safari, and Opera.
I remember, back then, my boss telling me to "only worry about IE" when writing a demo web application. One day I came in and my boss slaps a peice of paper down at my desk, wondering why a customer saw this on their screen. At first I was like "WTF" as it was a bunch of rectangles scattered around. The penny dropped and tried it in firefox.. same result. It was at this point I started to care about ALL main browers regardless of what anyone told me.
It is not as bad today, but my mind set is no different.
Should not be testing on just one browser.
Because lots of people who build websites only test them in Chrome. I use Firefox as my main browser, but there are times when I'm forced to open Chrome to do something (e.g. buy visitor parking permits from my local council) even with uBlock Origin disabled. As a non-technical user, if that happens to you even once, what motivation would there be for you to go back? As far as those users are concerned "Chrome works, but Firefox doesn't".