I won't dispute that some have been, but definitely not all pre 1989 cremations, with plaques at the DBC, (and Douglas residents too), are shown in the listings. I looked for three on the listings transcribed, as a test, and they are not there.
Please do not treat this as critisism; it is not meant to be. I think when one has personal knowledge, it makes sense to post it so that a note can be made by those who use the information. I hope this is useful.
Unfortunately, I do not have the time to go round and record the plaques, of which there are very many.
I suppose the deaths register is the ultimate check, at the Civil Registry.