One query isn't cheaper than two queries that do the same amount of IO and processing and operate in the same memory space

How is it the same IO?

Each query needs to navigate the index then read. The two queries do that twice.

Is it faster to read pages 30-50 of a book by:

a) Go to page 30, read until 50

b) Go to page 30, read that page, close book, open book, go to page 31 and so on.

Each page open you get to binary search to find the page.

Yes, (index) scans are rarely faster typical web apps.

Unless you have toy amounts data... or doing batch operations which is not typical (and can be problematic for other transactions due to locking, etc...)