Scott, it can be seriously intense! as with any technical discipline, it takes a lot of practice to get to “unconscious competence” and the lower HR that comes with it. Just like when you first started climbing and 5.9 had the heart racing, but it eventually becomes recovery level as skills and efficiency improve. Had my almost pro road bike only friend(really strong, 100mi+ stage racer type) get into mountain biking 6mo ago, and up until recently his HR would be well above AnT for large percentages of our rides while I was keeping it comfortably below AeT. This is despite the fact that he is more aerobically fit that me, with consistent 25hr(!) weeks on the rd bike(compared to my more ME focused 16-18hr of alpine/mtb training). Over the past few weeks he’s noticed a shift to more z3/tempo and less that 10% above AnT as he’s gotten more skilled and efficient with techincal riding. Still way too high intenisty to me, but road racers are hard as nails…
On the flip side, I have been able to drop from a 50/50 split rd/mt to 75% MTB for my bike training hours, while still keeping it below AeT. Probably more z2 that ideal for my volume of training, but my goals in the mountains are all local and not super high commitment, so I gladly take the enjoyment of Mt biking day to day at the expense of slightly non-optimal training practices. Plus, the mental training side of techy riding is great for scary leading! I’ve got so much better at committing to crux sequences from the do or die nature of MTB descending