I don't want start a doping argument on HN, and I don't dispute what you've written, but a very low heart rate can be a result of EPO usage.
There's an interesting account in (IIRC) Pantani's biography of riders having to sleep with heart rate monitors. An alarm would trigger if their HR dropped into a dangerous territory, whereupon they'd wake up and have to jump on a bike (which would already be set up on a trainer) in their room to bring their HR back up again.
Also in Obree's book "Flying Scotsman" [0] where his attitude to "medical assistance" makes him "not a team player", and recounts how he'd hear certain teams running through the hotel at night to avoid the potential blot clots associated with EPO usage.
Realistically, it's probably a "yes, and" situation. You don't get to that level without having a lot of natural talent, but... those years were what they were.
that's not why they had to get up. It's because they were hitting the EPO so hard, their hematocrits would be like over 60% (60% red blood cells/mL of blood) and thus when their heart rate/cardiac output would drop during sleep (to very low levels typically of highly trained athletes...not because of the EPO) the blood would be too viscous and start to clot
so the low heart rates caused a problem for those abusing EPO, but wasn't a result of using EPO
Reviewing this, I realised I was guilty of an assumption: that as a low haematocrit can absolutely cause a high heart rate, so a high haematocrit might contribute to a low heart rate... but I can't find any support for this supposition. :)
There's an interesting account in (IIRC) Pantani's biography of riders having to sleep with heart rate monitors. An alarm would trigger if their HR dropped into a dangerous territory, whereupon they'd wake up and have to jump on a bike (which would already be set up on a trainer) in their room to bring their HR back up again.