The word itself is perfectly applicable in various contexts that have nothing to do with startup companies; charities, political parties, bands, and internal projects. As such, referring to Buchheit as the founder of Gmail is reasonable.
I don't know, maybe it's an asinine point, but I don't think that's typically how the word founder is used, and it still seems funny to me.
Most laypeople think in terms of products, not corporate entities. The various international Google subsidiaries are all separate corporations, yet the lawyers & VPs who set them up aren't considered "founders" of them. Meanwhile, business units that operate with significant autonomy and generate a product that's distinct from the parent company's main business usually do have a "founder" figure.
Where would you put Android on this continuum? It's a business unit within Google now. However, it's run with a large degree of autonomy, has its own codebase, and creates a product quite distinct from Google. And moreover, it actually was founded by Andy Rubin as an independent startup, but purchased well before they got to market.