It was a theory, but one that cannot be proved or disproved. Nobody can say the exact lifespan of a hdd, ssd, nvme. Everyone experiences a difference in usage, trim amounts, optimization, Defrag etc.
For instance someone who uses a hdd, and only uses the same programs, never adding or deleting, will wear out the same paths faster than someone who uses more of the disk platters in different areas.
There's also engine theory, that starting an engine causes more wear on the rings that have lost oil in them due to gravity. With ssd/hdd same thing, startup procedures stress components more with loads and activity.
But with no way to accurately measure exact end-life, it's all a guess as to general expected lifespan.
Most ppl expect @ 5 years for a hdd before thinking about sector failures. Older SSDs didn't have controllers as good as later models and smaller sizes saw heavier usage comparatively in larger amounts as compared to larger size, newer SSDs, so mbtf is better with newer models.