The parent commenter stated that sum types work differently from a hypothetical float / NaN split, because compilers can't always "understand the code fully enough" to enforce checks. I simply responded that that is not true in principle, since you could just treat non-nan floats the same way that many languages treat non-null types.
Indeed, everything you're describing about non-nan floats applies equally to sum types - you can't operate on them unless you pattern match. You're highlighting the exact point I'm trying to make!
The fact that you consider this system "inconvenient", is entirely irrelevant to this discussion. Maybe the designer of Nim simply cares more about NaN-safety than you or I do. Who knows. Regardless, the original statement (that sum types and non-nan floats can't work the same way) is incorrect.