I don’t think exactly the same extent. In Minkowski space, if you pick an orthonormal basis, one of the basis elements will be timelike and the other 3 will be

I don’t think exactly the same extent. In Minkowski space, if you pick an orthonormal basis, one of the basis elements will be timelike and the other 3 will be spacelike, and the timelike one will be orthogonal to any linear combination of the spacelike ones, which, I think almost justifies saying that they are “90 degrees from one-another”. I agree that saying it is “space rotated 90 degrees” is, not really right. There’s no rotation that would rotate all the special directions to a time direction, that wouldn’t make sense. 1 ≠ 3 and all that. But there’s a bit of something kind of right in it.