There are some possibly inaccurate and definitely confusing statements in the reply you got, but the first part, that agm is a physical structure of the lead acid battery that can be tipped over without making a giant mess and that deep cycle is another function of design as opposed to a function of the lead acid chemistry is correct.
What’s left unsaid is that lead acid batteries which are damaged and not working right anymore have a much safer and lower tech recycling process than lithium ones do and that’s saying something because one of the parts is lead!
The safety thing is 100% true but only part of the picture.
E-bikes don’t need maximum energy density because they’re not gonna be used for long trips and are significantly lighter than cars and trucks.
China has many, many more electric vehicles than any other country and a ton of electricity production to run them. At some point it’s gonna become important to save the lithium batteries for the stuff that needs that high density power.
Maybe these better chemistries that will replace lithium are just around the corner. I certainly don’t count unhatched chickens.