can I be a bit more pedantic and say that perhaps flip-flop rear hubs (with different cog sizes) shouldn't be considered single speeds either.
I mean the ability to change gear ratios is there, albeit not in any practical manner.
if you allowed flip-flops to be considered single speeds, then riding any geared bike around without touching the shifters should be considered single-speeding as well

Then again, if you rode flip-flop hubs without carrying a wrench with you, then I guess it's ok

/ok i'll stop being a dick now.