Perhaps.

My grandfather had Parkinson's, despite living a very simple and plain life with an excellent (and calculated) diet.

I don't think they are suggesting that oral bacteria is the only cause of Parkinson's, but perhaps I misunderstood it.

was he exposed to Roundup?

Probably not specifically Roundup. He worked in rural electrification (farm stuff!) but was retired by the time Roundup became popular.

His parents were farmers, though, so there's no telling at this point what kinds of things (if any) he was around as he grew up.