> 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.

Yes, this is true, but they framed this as "the Postgres approach is better when you have a good schema design", but that's not true. There are plenty of ways the MySQL approach is better even when you have a really good schema.

> 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.

The point I was trying to make is that going to disk is going to be orders of magnitude slower than doing an in-memory B-tree traversal. Because of that, the cost of doing an extra b-tree traversal to find the page you're looking for is a relatively small cost compared to reading the page in the first place