Hi Apollo .
Yes, there is a good chance that the snails came with the duck weed. I got them in my 1st system that way.
I've seen others end up with them, without duckweed.

The only problem that I watch for is...they like to get stuck in the in-take of the water pump... . They are difficult to get rid of. They seem to lay a ton of eggs...and they are hard to see.
I sometimes crush a few and toss them in the fish tanks. I have a friend who has aquariums. He comes and collects them for his 'puffers'.