Actually it's not splitting hairs, it gives the sentence a different meaning. 🫣

I agree....I was just thinking he may have used the 'must' of the first part: the 2e must be or must appear.....
Splitting hairs :-)

Hallo Maryse, 'or appears to be older' is correct.
If you add must, then it would have to read: The second one has to /must appear to be older than the first. That is an instruction of what the second one has to look like / or appear as, whereas I understand it to be describing what the second one looks like, and he looks like he is older. The way he looks, he has to be older, he must be older, or at least he gives the appearance of being older.

or could it be : must appear ?

Just the absence of an 's' 😀 - The second one must be, or appears to be older than the first. (appearS)