That spammers call some of their spam callbacks doesn't change the fact that "callbacks" to forged addresses are both unsolicited, and sent in bulk (multiple copies substantively identical).
Callbacks are just a way to reflect your spam problem to innocent victims without reducing the spam volume at all. That's extremely anti-social behavior.
Yes indeed, especially given that you know 80% or more of alleged sender addresses are forged, callbacks are spam.
So don't send them. Furthermore, block those who do, until they stop the spamming, just like you do any other spammers.