I can't say it did or didn't cause your problem, but I don't see any point in leaving any kind of tender or trickle charger continuously looked up to a battery for long periods of time. If nothing else, having anything plugged in unattended, even a coffee maker in your kitchen, is a potential fire hazard. That's my Fire Safety speech for the day.
My unsolicited recommendation, from a standpoint of safety only and not function, is to just hook up a Tender once a month or even every 2 weeks if that suits you, but just get the battery topped off then disconnect everything. The BT will shut off and on by itself, but you've still got everything plugged into an outlet. You have to keep a refrigerator plugged in, but you don't have to keep a battery tender plugged in.
I doubt it was the Battery Tender (if a true Battery Tender unit) that ruined the battery, but instead you just got a lemon battery as to me the second scenario is much more likely.
I have no proof that a "smart" charger won't ruin an AGM battery but am confident enough in my presumption that somebody would have to conduct some kind of test or study and prove to me that they will.
I'm not saying that BT is the be-all end-all of smart trickle chargers or that they are the only acceptable brand, but there are others I've seen that I wouldn't trust and I wonder how many people who claim a "trickle charger" or "battery tender" ruined their battery were using some kind of off-brand unit that is not of the same quality?
To the best of my knowledge, a "trickle charger" continously applies a charge to the battery while a "tender" monitors the battery charge and will shut off then it's reached a certain level then kick back on when it drops below that.
I think a lot of people hook up a trickle charger and leave it on for months and I do believe that will damage a battery.
Not saying that's what happened here, but I think it's part of the widespread opinion that a "tender" will damage a battery if left on too long.