This seems to support the author’s argument, rather than weakening. If we don’t identify counterfeit bills quickly enough, those bills proliferate, so we need a practice that catches those bills immediately (like special ink).

Some counterfeiters can perfectly replicate the “special ink” practice, which defeats the purpose of adopting it in the first place. This undermines the argument by denying one of the author's assumptions.

If anything, this gives even more reason to switch away from the microprinting practice, because photocopying technology will soon render microprinting useless as a defence against counterfeiters. This doesn't weaken at all.

Generally, people who create counterfeit bills aren’t committed specialists. What does that mean for the argument? Can hobbyist counterfeiters imitate either of these techniques? We don't know, so this is irrelevant.

In other words, the benefit of catching counterfeit currency early is uncertain. This does nothing to the argument, because it neither strengthens nor weakens the case for the easily-detected special ink.