Very true! What's a slow burn is very much up to the genre/fandom/format. I've seen fairly short fics (word count wise) tagged as slow burn since the relationships develops over several short scenes over a long period of in-story time, and while I don't think that's "wrong", I'd myself not use the tag unless the fic itself is a "long" one.

And yeah, the Good Omens fandom is All The Kinds Of Slow Burn XD It's like an ice cream bar full of different kinds of "watch these two people slooooowly get together" XD