Lawyers sending letters to discourage actions they do not like are fairly standard. I've had attorneys tell me that if you are not getting letters like this, you aren't making enough of an impact. And to be clear, this is just a letter - tossing one of these out just to see if it works is an easy tactic because many smaller organizations are terrified of litigation, and will cave to demands even if there is no legal basis for them.
Do take the letters seriously... determine whether there are valid legal claims presented. But if there are not, it is a scare tactic, so don't stress over it.