Point of terrorism is achieving a political change through terror. Terror is not the end in itself.

From this point of view, mailing of anthrax in 2001 was very inefficient.

We need to expand the set of possible actors and goals. The anthrax campaign happened immediately after 9/11. The result of the campaign was the end of effective political opposition to the massive expansion of military and "secret" aspects of the USA government made possible by 9/11. Cui bono? Certainly not the several implausible patsies the authorities immediately had lined up to blame.

Numerous USA calamities make a lot more sense when we study them with an eye toward who benefits rather than who is blamed.

What made 9/11 such a successful terrorist attack was that it made the US live in fear. That is a change (and it hasn't changed back). Terror (fear) is a perfectly fine end in itself for terrorism.

The anthrax mailing caused changes and fear in US society with very little work. That sounds like a very efficient terrorist attack IMO.

Terror only propagates through mass media. Mass media is the issue.

Given that an attack of this sort would be propagated through mass media, then the claim that the attack would be ineffective is wrong.

ineffective to kill millions which was the initial claim.