They look like it to me. The leaves should have a dull waxy appearance with pronounced veins, which these do. Also the blooms are developing deep coloration as they mature, and the petals exhibit a bit of serration. The size difference in blooms is a fluke.

Note: those 5-gallon pots are fine to start with, but MG are no means dwarf shrubs. If you intend to keep these in containers for an extended period, I would gradually work your way up to 20-24"+ containers, which equate to about 15 gallons. Various outlets sell such containers for about $25 but you can get them for less than $20 on sale or clearance. Once loaded you can move them around with carts/dollies/wheelbarrows. I let mine dry out a touch in the garage during the off-season so I can "cheat" and carry them around.

Springwood, thanks for taking the time to post and identify these plants. I like knowing the correct name.

I'm not sure they'll survive winter here in Brooklyn NY so I may not get to move them to bigger containers. If they do make it, I'll certainly be on the lookout for larger pots.

If you have an unheated garage or shed you can move them to, they should winter over just fine in your zone, based on others' experience with various cultivars of big leaf hydrangeas.

Thanks, NHBabs. I'm hoping for the best. They're gorgeous & I keep on running out to my back deck to look at them!