All you're doing is having the solids removed

Actually they pumped out the entire contents: solids, liquid, and scum.

They pumped out all the scum first by holding the hose at surface level
to skim it all out.

Then they sucked out the liquid until the solids became visible.

Once the solids were visible, they sucked that out, using the little
bit of liquid left to help carry it up the hose.

When they ran out of liquid and there was still some solids left, they
had me turn on the kitchen and bathroom faucets to introduce more water
into the tank to mix with the solids so that the hose could continue to
pump it all out.

When they were done I had a pretty clean dry tank.