Agree with your assessment.
While true you can take any old soho wifi router and just use its AP features... soho wifi routers, include a routing function, switch ports and a AP in 1 box..
The switch is dumb, and therefor almost always the AP is dumb.. Or atleast configured that way via the native firmware. Other then allowing "guest" ssid that is not bridged to the switch ports.
While the hardware quite often, but not always does support doing vlans. I have never seen the native firmware leverage them for anything other than maybe the "guest" network most of them allow you to create, which really is just not bridged to the switch ports vlan.
Normally they do actual use vlans, this is how they isolate the wan and the lan. But the interface doesn't allow the user to change or manipulate them really in any way.
So simple way to just use them as a dumb AP is just turn off dhcp on them, and connect them to your network via one of the lan ports. Now anything on wifi (not guest ssid) is bridged to your switch ports = AP..
So if you want to actual do vlans, either put 3rd party firmware on the device to expose way to configure the vlans. Or get an actual AP, then yes normally would support vlans..