I suppose if you use old Reddit and block ads they're not making money directly but it's still traffic.

I always used old Reddit anyway, I left because it turned to shit not because of a particular app.

Isn’t that still traffic and aren’t they still getting money?

Yes

Is it less money somehow?

No.

Dunno for sure, I feel the same way as you, but I think it's more about "I refuse to use the app you intended me to be forced to by killing [favorite 3rd party app].

If combined with an adblocker they don't get your ad revenue but they do still get to add you to the tally of "active users", so I still feel abandoning ship altogether is best practice.