ISAK is the international standard on body measurements -...

ISAK is the international standard on body measurements - International Standards for Anthropometric Assessment.

Otherwise you measuring your waist, and then me measuring it will be entirely different.

You can take a formal course to qualify and use it professionally, for example - https://www.essa.org.au/epd/isak-level-1-anthropometry-courses-exercise-research-australia/

Hence the proposition that apps like this are more accurate, taking out human error and environmental factors. The margin of error is statistically much lower, and consistency much higher.