I think OP is just alluding to the fact that Postgres needs to do less work to go from secondary index to table data, since the tid is a direct pointer to the exact page and slotted entry while MySQL needs a b-tree walk.

> primary key index will be mostly cached so the cost of the indirection is much smaller than it may first appear

Not sure I follow. If it's in-memory you save having to read from disk, but you still have to walk the b-tree to go from PK to data.

MySQL was generally (pre 8) optimized for point queries on primary keys. So rows are stored in the PK index, the PK index is a clustered index. Everything more or less falls out of this.