For an individual, yeah. Apple belongs to a number of standards bodies and consortia, though; when they deviate from an existing standard, they usually do do more deliberately than this letter implies.

I was not trying to get in contact with them in my capacity as an individual.

In that case, a simple line saying "we tried to contact you to no avail" would have avoided looking rude.

I just don't get why you're so stuck on this one point on etiquette. To me, saying "We tried to contact you to no avail" is ruder than omitting the line.

An open letter is obviously not a communication to the recipient, but to the public. It's a rhetorical exercise you use so that the public will put pressure on the counterpart; hence it has to win the public to your side. In order to have the public on your side, you have to be as clear as possible about your motives and your methods - otherwise you'll get people questioning them, which is exactly what you can see in this thread. That's a fail. A simple paragraph saying "we tried to engage with you but could not because X" would have made the writer more sympathetic to the public, imho, and avoided most of the challenges you see here.

If that looks rude to the counterpart, it doesn't really matter; everyone knows any politeness is fundamentally insincere anyway, because if one had not wanted to ruffle their feathers, one wouldn't have published anything in the first place.