40-70min comes from the range in how long included athletes could hold FTP for in the one Seiler paper.
That time interval is not derived from a fundamental physiological principle, but rather is just the range of times seen in that sample of athletes, in combination with empirical data experience from those guys’ careers coaching professional cyclists.
It is not immediately obvious that the results seen in those athletes would be externally valid to recreational cyclists.
I personally don’t know of any data in non-professional cyclists that looked at TTE in recreational cyclists, other than that one I linked.