If a SIMD ISA exists, someone must know that it exists, because definitionally we only apply the term "SIMD ISA" to things that were consciously created to be such. So we could simply check every such example. Saying "only known example" is indeed silly.
But e.g. in mathematics, if we say that "almost every member of set X has property Y; the only known exception is Z" then there absolutely could be more exceptions, even if we pool the knowledge of every mathematician. It isn't necessary that X is finite, or even enumerable. It could be possible for exceptions other than Z to exist even though every other member of the set that we know about has the property. It could be possible to prove that there are at most finitely many exceptions in an infinite set, and only know of Z but not be able to rule out the possibility of more exceptions than that.
We don't even need to appeal to infinities. For example, there are problems in discrete math where nobody has found the exact answer (which necessarily is integer, by the construction of the problem) but we can prove upper and lower bounds. Suppose we find a problem where the known bounds are very tight (but not exact) and the bounded value is positive. Now, construct a set of integers ranging from 0 up to (proven upper bound + 1) inclusive... you can probably see where this is going.
The latter doesn't apply to SIMD ISAs, because we know all the interesting (big hand-wave!) properties of all of them rather precisely - since they're designed to have those properties.
But e.g. in mathematics, if we say that "almost every member of set X has property Y; the only known exception is Z" then there absolutely could be more exceptions, even if we pool the knowledge of every mathematician. It isn't necessary that X is finite, or even enumerable. It could be possible for exceptions other than Z to exist even though every other member of the set that we know about has the property. It could be possible to prove that there are at most finitely many exceptions in an infinite set, and only know of Z but not be able to rule out the possibility of more exceptions than that.
We don't even need to appeal to infinities. For example, there are problems in discrete math where nobody has found the exact answer (which necessarily is integer, by the construction of the problem) but we can prove upper and lower bounds. Suppose we find a problem where the known bounds are very tight (but not exact) and the bounded value is positive. Now, construct a set of integers ranging from 0 up to (proven upper bound + 1) inclusive... you can probably see where this is going.
The latter doesn't apply to SIMD ISAs, because we know all the interesting (big hand-wave!) properties of all of them rather precisely - since they're designed to have those properties.