Yet we both agree that S stands for second, doesn't represent plural, and shouldn't be removed for singular.
That's something, I suppose.
Actually, I think it can stand for the plural, assuming a device's capacity is greater than 1 tera FLOP(s). You are dealing with an assumed quantity of time. After, all, who, (in their right mind), would think is terms of a "teraflop", being measured over the course of a a day, hour, or even a minute.
The "second" is an assumed quantity of time, except where it would be absolutely necessary to maintain coherance.. To wit, "MPH, MPS, MPM". (*)
Besides, you're dealing with a real old timer. I still measure frequency in "cycles", instead of "Hertz" (**)
(*) In the case of "MPS", and "MPM", to be correct in context, you would have to write out, "Miles per Second", and, "Miles per Minute" at least the first time you used then, for clarity's sake.
This is fun, can we do this some more?
(**) EDIT: In the old days frequency was measured in, "cycles per second". This was from zero through infinity. Sound and AM radio would be in "kilocycles", (until you hit one millions cycles per second, then the term began as "megacycles". So, you have "kcs" and "mcs". When the measuring standard was changed to "Hertz", it was implied that the included time of measurement was one second. So saying, "Hertz per second", would essentially be a vulgarity, or unnecessary extension of the term.