Common misconception. That just means the algorithm for counting letters can't be as simple as adding 1 for every token. The number of distinct tokens is tiny compared to the parameter space, and it's not infeasible to store a mapping from token type to character count in those weights.
If you're fine appealing to less concrete ideas, transformers are arbitrary function approximators, tokenization doesn't change that, and there are proofs of those facts.
For any finite-length function (like counting letters in a bounded domain), it's just a matter of having a big enough network and figuring out how to train it correctly. They just haven't bothered.