Then perhaps a comment along the lines of "This is really neat! I'm so glad to see someone else doing this; I originally patented something akin to it and then (never used it/did and here's the thing I built), and it's awesome to see continued development in this area".

As it is, it's not clear what your intent is (hence my question). You aren't congratulating them. You aren't expressing excitement. To your credit, you also aren't claiming they're infringing, though it begins to approach it.