The relationship between tedium and difficulty (as referenced many times on this forum) is a precarious one.

Essentially, I see it like this: Time can be a form of difficulty... but is not synonymous with it

For instance: Lay flat on your back and lift your feet six inches off the ground... easy!!!

Next: Hold that position for 4 hours

It doesn't require any "skill"
It would indeed be tedious, but the one thing that makes tedium difficult... the one thing people have yet to reference at all... the one word that defines why that is hard....
Endurance
An endurance test can indeed be difficult. The problem is... in the spirit of balance, all difficulty cannot rely on endurance testing. If it is in game.. I'd like to see it used sparingly, and not overdone on every piece of content
If there were a 16 hour fight... it would be difficult... strictly based on the time it took.
We are definitely in an "Instant Gratification" society
and also... Many people would prefer a video game not be an endurance test
I can definitely see both sides
But try to avoid saying tedious isn't difficulty.. when, in fact, it is... just not the difficulty we want to see.