It doesn’t use the max HR but the average HR for each bucket.
Each ride has a power vs HR curve created by breaking the ride up into 1 minute segments, adjusting for HR lag and calculating average power and HR for each segments. These are the data points used for the decoupling related charts on the activity power page. Only segments with 100% moving time are kept.
The traces for the comparison chart are created by bucketing the power-HR segments for all matching activities into power buckets and averaging the HR.
Hope that makes sense!