Ahh, you're right. I thought I the count vs measure comparison document at:
- FreqCount Library, for Measuring Frequencies in the 1 kHz to 5 MHz Range
- FreqMeasure Library, for Measuring Frequencies in the 0.1 to 1000 Hz range, or RPM Tachometer Applications
...was the same exact document, and the table at the top was the same. But it is not. On the Mega, FreqMeasure runs the Timer4 counter fast based on a clock, and uses the pin49/ICP4 to InputCaPture the Timer4 count when each cycle happens, versus FreqCount using 47/T5 Timer5 external clock pin to count the number of cycles in a certain interval of time. They are completely different mechanisms and, on the Mega, use different pins (and different timers).
Are you looking for something like a tone() example?
https://www.arduino.cc/reference/en/language/functions/advanced-io/tone/
You could add tone() and a jumper to one of the FreqMeasure or FreqCount examples. (Or to one off Nick Gammon's examples)