Many houses (including mine) were built pre-3 prong outlet. i.e. old crap. Lots of times (such as in my house and in my garage) 3 prong outlets got installed at some point but are not actually wired with that ground wire. This old wiring heats up easily, tends to be shoddy, and is straight up a fire hazard on anything drawing even 12 amps regardless of what amperage the circuit is. Just because someone replaced the fuse box at some point and put a breaker panel in and there's a 30 amp breaker, doesn't mean the wiring supports that. Our garage had a two prong outlet wiring with a 3 prong outlet (obviously 3rd prong/ground not hooked up) with a 6 outlet plugin stuck into that outlet. i.e. up to 6 different devices plugged into one incorrectly wired outlet. Which reminds me....yeah...people are way more likely to hook a power strip up to the level 1 outlet and run half a dozen things off the same outlet where one of those things is the level 1 charger. I can imagine a scenario where someone runs a 50 foot cheapo indoor extension cord out of the garage and uses the level 1 charger plugged into the car sitting in the driveway. That's not going to happen with a level 2 outlet.
So yes there is exactly ZERO logic that says the level 1 charger is not as good for your battery. Complete nonsense. It is, however almost certainly less of a fire hazard to use level 2 because yes there are fewer possible ways to be a complete idiot.