I guess it starts downloading directly. The entry is, I presume, for possible future 64bit-only Plus! releases. Not that I'm saying there will be any, I am really just having a guess.
That is, if Patchou would release a 64-bit Plus! and someone attempted to run it on a 32-bit Windows, then the message would show up (and would download a 32-bit version which would of course exist)