Why the pre-dreadnought label looks backward

Does calling older battleships pre-Vanguards expose the same problem as pre-dreadnought?

Yes. Both are retrospective labels. Pre-dreadnought became useful after HMS Dreadnought appeared, but it remains historically awkward when applied to the preceding era.

Pre-dreadnought defines earlier battleships by a development their designers could not anticipate. The term appeared comparatively quickly and was in use during the First World War, where it could distinguish older battleships as a group, but that does not make it a sound label for the preceding era. The pre-Vanguard joke exposes the same backward-looking logic by applying it to ships that nobody normally describes that way. The exchange shows how a later technological break can distort the category applied to earlier warships.