I haven't opened one myself since the 5S but that sticky substance that holds down the battery, while it might not be "glue" in a strict technical sense, adheres the battery to the case rather firmly. It's totally reasonable for someone to say that's "glue". This is especially true if they end up deforming the battery when removing it.
I think it all boils down to reparability. Just doing the basics (disassembly, and part replacement) with these assemblies take A LOT more practice and skill than the vast majority of people are generally prepared for.
Does it have to be this way? I think that's the key question. There's ultimately trade-offs between having something that's easy to work on vs something sleek and thin vs something that's affordable vs something that's performant. As consumers we are at the mercy of "genius designers" to decide those trade-off's for us. Personally, I don't mind having my phone be a nightmare to repair as long as it does the job I need it do and I can pay somebody to fix it if that's prudent. With other things, like appliances (that don't have borderline microscopic parts), I really want something that has a service manual.