And you're likely going to spend years trying to get rid of it too eventually..... (The voice of experience here....!)

I really like the perennial sweet peas, and you described the flower perfectly as "hardy." In some areas they can be invasive but not everywhere. My hunch is that if it took 3 years to get off the ground it won't become a pest overnight.

Anyway, your person at the nursery was obviously incorrect, but that happens. I always like it when they are really condescending at the same time as they are saying stuff that's wrong. :)

Lathyrus latifolius is the perennial sweet pea, and the annual is Lathyrus odoratus. The perennial doesn't have the lovely scent of the annual, and here the perennial sweet pea is a real problem. I am still removing random seedlings more than 10 years after deciding that they were a bit too enthusiastic for me, but happily their spread was limited by the drive and walkway. I sometimes pass a field of a couple acres that has been totally taken over, so hopefully yours will be more restrained.

You should be able to grow more of your own easily when the seed ripens. Fortunately I think that's the only way they spread (no runners or suckers as far as I know). It spreads a bit here too. Comes in pure white also as well as shades of pink, something to keep in mind!