When I delegate HP to another account, say the HBI bonus units, that value stays the same but still counts for the accrual of HP with the APR% that the witnesses decide on, right?

HP is an abstraction. At the blockchain level, there are VESTS (sometimes referred to as MVESTS), and there is a reference rate. When you power up, the HIVE is converted to VESTS at the reference rate, and when you power down, the VESTS are converted back to HIVE at the reference rate. The APR% for accrual of HP is in the reference rate, so any HP in your account (even if delegated) will still receive it.

(Actually this creates slight incremental benefit for Hive SBI for long-term delegations, because the bonus units are calculated at 1 bonus unit for each 2 HP delegated at the start of delegation, and then removed at the end. We don't update the bonus units as the HP value of the delegation increases (although you could force it by redelegating each time the HP value goes up by 2 or more if you really wanted to capture that in bonus units)

It's not a lot, because bonus units from delegation are only about 5% of total units...