What he’s saying is that Safari has pretty good standards support, even if it doesn’t run every wizzbang new standard. Maybe you don’t remember the bad old days of IE, but stuff written for IE was horrendously broken. Web developers had to put in all sorts of fixes to maintain support for IE (which was critical, since it was still the most popular web browser at the time). However, a page that’s written for the set of standards that Safari supports will run well on modern versions of Chrome and Firefox, even slightly older pre-Chromium versions of Edge.
I do think you’re being deliberately obtuse to an extent, though, or at least overly literal. No one is suggesting that targeting Safari will support every browser that’s ever existed, but what he is suggesting is that targeting Safari (or the set of standards Safari supports) will give you coverage of every browser with a share greater than 1% or 0.1%, basically for free. While targeting for Chrome lets you shoot yourself in the foot with technologies that aren’t well supported in Firefox and Safari.
The big thing missing in all this is to what extent adding Safari support breaks an existing web page. Back in the IE 6 and 7 days, it often took just as long to get a page working in IE as it did to get it working in all other browsers combined. That doesn’t seem to be the case with Safari, it performs to standard, but it doesn’t support every latest standard. Which is a slightly silly thing to complain about, at least in terms of JS, since most people are still using things like babel in their project boilerplates. It’s also somewhat silly to see how eager web developers are to live on the edge, in terms of supported technologies. (Here’s a hint, most developers don’t get the privilege of working with the latest and greatest technology. I still have to write in Python 2 compatible code on occasion and maintain VBA code!)
Edit: And lazy devs who just want to target Chrome are a legitimate issue for the longevity of the WWW. Basically, you’re complicit to the act of giving one vendor (and a big one, at that) control over the web again, just like back in the Netscape 4/IE 5 days. Do you legitimately trust Google any more than you trust Microsoft? I’d suggest that you shouldn’t. You shouldn’t be an enabler assisting one firm to have a 90%+ market share. Oh sure, Google may not let Chrome stagnate like Microsoft let IE, but giving Google control over every major browser lets them do some big self serving things, re web standards. And, if Google bullies the Mozilla project into siding with it (by threatening the Mozilla project’s main revenue source, for instance), then it can artificially manufacture consensus among two of the three web browser developers.