It's an R&B album made in an era in
>which R&B had not fully shaken off much of the glitter from
>the night before's disco...

But couldn't that be said for alot of albums and songs from the era?

A lot of so-called Disco records were simply R&B records that happened to be coming out either at the beginning...during...or at the end of the so-called Disco era? R&B records that appealed to DJs who were trying to keep people on the dance floors......aimed at charts that included so-called disco records....

I mean what would make Off the Wall any more or less a Disco album than say one of Heatwave's albums?

The classification can become very subjective.