Step 6: what does "leave it" mean? Simmer after boiling, turn heat off, or keep boiling?

Agree with Emmie, especially if you look at recipe's step #5, which has the same instruction.
But I'm a bit worried about the soup possibly boiling over during 20 minutes.
Just for your comparison and/or use, here's another similar recipe that has more precise instructions. You can use or omit the cute presentation in a whole small pumpkin.
https://www.meilleurduchef.com/en/recipe/pumpkin-veloute.html

Yes, personally I'd recommend just simmering it.

Thanks so much for the speedy reply Emmie! I'm used to recipes that recommend at least the occasional stir if something is going to be boiling for 20 min. I'm going to take the simmer and stir approach after boiling! Great community here!

Thank you so much Nancy! Wow - that bounce over to the other site opened a whole new world! I was going to improvise with the recipe to use up some things on hand, so that other recipe gave some more ideas for adding complexity! Wonderful community - made my day!