I too think the thesis is flawed. It's true that for a given power output, higher cadences will tend to generate a higher HR than lower cadences. But it does not follow that declining maximum HRs are a factor, because in endurance cycling max HR really isn't terribly relevant

Plus, when doing TT-type efforts I tend to find that I adopt a lower cadence than I would for a longer ride. On the latter I am likely to spin along at in excess of 90 rpm, because while that is energy-expensive, it saves my legs. Time-trialling for an hour I am likely to adopt a cadence lower than that, because alough my legs may be trembling with exhaustion at the end it allows me to manage my effort more effectively and not overcook it - or so it seems to me.

All that said, here goes:

Solo for one hour, >23 mph, cadence c.75
Fast group ride 25-26 mph, cadence > 90
compact 50/36
Cassette 12/25, 10-speed
Age 58