In order for the speed limiter cutoff (as opposed to the redline/engine over-rev cutoff) to work off the engine speed, the computer must know what gear you are in. It would also need to know the gear ratios and the final drive ratio. In the case of the 5-speed manual transmission, that would require switches or sensors to tell the computer what gear the transmission is in, which is a lot of additional complexity and cost for NO benefit when there's a perfectly good VSS to use for speed information, which is already wired to the computer. [1]
In the case of the computer-controlled automatic transmission, the computer always knows what gear it's in, but the VSS is still used for the cutoff.
[1] The VSS is used by the computer to provide a dashpot function where it sets the rate at which the idle air valve closes when the throttle is closed, to prevent "lift throttle snatch".
If the cutoff did get it's information from engine speed, tire size would not change the relationship between speedometer displayed vehicle speed and engine speed. Only a gear ratio change or final drive ratio change will do that.
