That argument has been discussed many times here and elsewhere. It is based on the common usage of the word ‘theory’ as equivalent to ‘wild guess.’ Even a scientific hypothesis is the result of observation and application of current knowledge about the phenomenon at hand, not just a wild guess.
The closest thing to a ‘law’ we have in evolution is that a species must adapt or go extinct when its environment changes. Since we can never predict which species will go extinct or how it might adapt, evolution will always be a theory, as defined by science. That does not give the ToE any less weight as an explanation for the change we can observe in the fossil record.