That's a fairly cavalier attitude to a mater that can get very costly. Scare letter or not, this is definitely a case where you better err on the side of caution.
If they were serious, they would have sent more than an email.
Their attorneys did draft the letter. I'm not sure how serious they actually are in this case. They're claiming that we dilute their family of marks.
You should take this seriously and talk to a lawyer about this.