If this excuse is poor, what qualifies as a good one?

You want Avast! to be 100% super-happy-honest with its marketing, but this is the age of Puffery, and if you want to excel as a company, you need to be on-board with this idea or gtfo.

90% of users who saw a statement like “This AV Can Not Detect All Virus, Use At Your Own Risk”, would run to whatever AV told them what they want to hear, i.e. “You are safe now, we will protect you”

So your problem is not just with Avast! it seems, but the whole state of online security in general? Well, you are not alone there :wink:
Avast! has an established email (virus@avast.com) for submissions, were you not aware of it?