Not exactly sure what claim you’re specifically referring to. However, when discussing the question of whether evolutionary theory is or is not true, calling something a “transitional fossil” is an example of the fallacy of begging the question. That is, using a “transitional fossil” as evidence for common ancestry or Darwinian evolution is a circular argument. It assumes the very transition you are trying to prove.
How do we know whales evolved from land animals?
Because there are transitional fossils such as pakicetus.
But how so we know Pakicetus to be a transitional fossil?
Because whales evolved from land animals, and Pakicetus fits within that transitional pathway.
If some fossil is determined to be transitional due the fact of evolution, it can’t then be used to establish the truth of evolution. At best one can only say it is consistent with evolutionary theory, and fits with evolution’s predictions, which may well be a significant observation, it it can be declared a “transitional fossil” and then used to establish the truth of evolution.
(Sorry, meant to address this to the original post)