Yes, you are correct.

If you’re using your aliasing-service to “blend in the crowd”, just like how TOR works, you may not want to use a custom domain.

For me, the purpose of AnonAddy is first, and foremost, to help me combat spam. Any privacy improving aspects I see as purely bonuses.

I will use the shared domains from time to time though, if I consider the risks to be too high. This goes from posting an email in a public forum, to signing up for a particularly suspicious newsletter for one-time benefits.

As always, you should take your own threat model in mind.