That’s an unhelpfully restrictive definition of illusion that is itself illusory. An illusion is also:
A sensation originated by some external object, but so modified as in any way to lead to an erroneous perception; as when the rolling of a wagon is mistaken for thunder.
The text is hidden or revealed through a change in perspective. That is the illusion.
It’s an optical illusion. By definition their isn’t generally anything YOU would call erroneous about any optical illusion, I’d guess. The fact that the text is difficult bordering on impossible to read at some angles is the perceptual error. Stop ignoring obvious interpretations to support your pedantic trolling.