I agree that he goes a bit over the top and that in the end it doesn't really matter, but one space does look a lot better.
As with most things, whatever you're used to is what will look better to you.
Also the writer of the article sucks. The piece is titled (in part) "Why You Should Never, Ever Do it" but he does not actually explain why you should never, ever do it, as though it were some kind of horrible faux pas. He points out that "modern" style guides use the one-space rule but does not mention that for a long time, two spaces was in fact the rule. He says that "Most ordinary people would know the one-space rule" and completely ignores the fact that two generations of people, most still living, grew up with a different rule. The two-space rule wasn't just some brief quirk in history, as he claims. It's how people typed ever since people started typing. Because, you know, typewriters didn't exist before then.
People tend to think that how things are now is how they've
always been, but that's simply not true most of the time. Intelligent people understand that.
I have no idea who/what Slate is, but I'm not impressed.