Honestly, that sounds like the most sensible advice. If I run a small US-based business and I receive one of these letters, I'm almost certainly just going to ignore it--as I'm sure many are already doing. It's not like I'm ignoring an official government notice. It's just an email from someone random making an assertion/request.

Maybe I put a notice up and geofence EU IP addresses but it seems that would just raise my visibility and suggest that I think I'm doing something wrong (whether or not I am).

At the least I'd wait for some indication that a random US ecommerce site (or whatever) actually has something to worry about.