Normal.....some years the seed load is very heavy, initially at the expense of foliage.
It can be a sign that the tree is stressed for some reason, especially if the foliage doesn't fill in nicely later. Depending on the size of the tree, there can be very little you can do to alleviate the stress.

Your local nursery was correct.

I've noticed that here in Minneapolis. Many of the neighborhood silver maples are a bit sparse and just loaded with seeds. We've had several years of very hot and dry summers. Perhaps that is affecting them.

Seeing the same thing with our silver maple in our front yard. Nursery guy said it was stressed (tough summer and winter). We've got galls already too (ugh!). Nursery guy said all we could do to help was to water it. I've been a little stingy with the irrigation this year so it looks like it'll be getting a good soaking over the next weeks. Hope yours gets better as well, good luck.