Only if the cost of making a better counterfeit exceeds the cost of the legitimate item. It's a t-shirt, it doesn't have a chain of attestation.

It doesn't cost much for me to write my signature, but people can still detect forgeries.

Unless the counterfeit shirts were manufactured in the same factory, with the same process, I don't think they'll be identical. Dries Depoorter might be adding the numbers herself by hand, so unless the counterfeiter has the same tools as he does, there will likely be differences.