I don't think your Mom is an OW. There's no evidence that she is.
These days a dating relationship seems to automatically infer the relationship is exclusive. If your Mom is at least 60 or older, she grew up with a generation that dated more than one person at a time - it was not considered cheating. Unless the guy gave you his high school ring or his college frat pin, you were just dating, you were not committed to dating only each other.
I know several people over the age of 60 who have long term dating relationships with more than one person. It works for them and there's no reason for me to judge.
Yes, I think you were overly sensitive concerning your Mom's dating. The man she is dating isn't married to anyone else, you have no evidence that the other girlfriend is in the dark and thinks she's in an exclusive relationship. As far as you know, she may be dating two other men.
When you're single and in the later years of your life, friendships, dating, girlfriends/boyfriends do not necessarily have the same rules. Companionship sometimes is more important than the person being exclusive to you.