I would say that
we use a comparative degree when we compare two people, animals or things
Is a gross oversimplification and it is grossly incomplete.
I am fatter now than I used to be.
I am comparing, but there is only one person involved.
(There are still two agents in the comparison, namely "me now" and "me before", but stating you need two people, objects or animals does not seem to allow for one person to appear in two roles.)
The whole idea of strictly counting the number of agents in the comparison seems strange.
Of course the sentences
A is taller than B and C
John is taller than all the other kids in the school
In the second case, yes, you can say that "John is the tallest kid at school"
But I can also use the superlative with less than three agents:
John is the tallest of my two friends.
And then we haven't covered sentences like
I feel better than I ever did!
I think this is an example of educational books trying to teach tricks instead of rules, and they are one source of frustration to learners that advance enough in the subject to realize that the "rules" they have learned were nothing more than incomplete tricks or "rules of thumb" at best. (Notice the superlative "best"?)
Similar "rules" that get irritatingly confusing (and I have seen them):
Never use a comma before "and"
Never start a sentence with "as"
And the worst - but off-topic as it is not about English:
Don't worry, Dutch does not have any grammar.