The header image on the website shows slashed zeroes, but the font preview doesn't, which is unexpected. In fact when previewing in Google Fonts there's barely any visual distinction between uppercase letter O and zero. There's only a very slight difference in width which you actually have to look for. Punctuation isn't centered in their box and leaves so much tailing space the test text `5240N 15:01 .84 8,9` reads like `5240N 15: 01 . 84 8, 9`
These problems don't exist as much in the variable width font, where there is no tailing space in the punctuation and the uppercase Os are rounder.
There's also some weird pinching in the corners of N. 1, l and I are perfectly distinguishable though, so at least it's got that going for it.
The hollow zero vs slashed or dotted zero thing is often done in fonts with an extra variant; basically the font can contain both and will display what the application requests. So you should assume that the font really does have the slashed zero, but the font viewer you looked at it with didn't ask for it.
That's a good point. I added that rule (font-variant-numeric: slashed-zero;) to the font preview using the browser's devtools, and I saw no difference.
And aviation still is full with it. For better or, in my opinion, worse... but aviation is so stuck of outdated and inconsistent crap in general...