When it comes to your original question, as far as I know the piece mentioned really isn't that difficult for someone playing intermediate stuff already.
But when it comes to difficulty of pieces in general this is how I think now:
IMO grades mean nothing much when using a creative learning method. They are for those who want to use a structured and scheduled learning method. Both methods can work fine, but in the former case jumping to something more advanced can distrupt the learning process.
When you take on a piece that is very difficult for you, you need to have flexibility in your method. First you have to have patience to spend as much time as needed with whatever you encounter. You may need to learn a new technique, read up on the style, use many different practice methods etc. You will often have to divert yourself from the actual music to solve problems.
Second you will most likely enter a state of diminishing returns at some point. At that point you must accept to let go. Let the piece rest even if not finished, take what you could manage this time and come back to it later when you have developed further. If you try to force your way through the piece even when not having what it takes yet, it will be difficult to unlearn the shortcuts you took and those things that didn't work. But if you take what you can from the piece at the moment and then leave it, you have already learned things that will benefit your learning of other pieces and when you return to this monster again it will be much more manageable. I have sometimes returned to a piece 2 years later to finish it.
I think it's a personality thing. Some people need to finish things and feel they are constantly progressing in order so they prefer to stay in their comfort zone and a little above it or just lower their expectations on quality when trying to learn hard pieces. I need to challenge myself constantly or I will lose motivation and I have learned to accept that some things will take a long time and I don't really need immediate rewards anymore. I also never could relate to structured learning or practice schedules but instead find it rewarding to constantly experiement and find new ways to approach whatever things I encounter.
I have seen many people end up quitting daily practice or playing altogether because they lost motivation when trying to jump into the deeper end too fast. It may be how majority of the people function. For me it was the other way round. The times when I actually thought I could not take it anymore and started feeling like not practicing anymore and even quitting lessons was when I spend too much time on studying pieces in the "right" order and choosing only pieces appropriate for my level. Whenever I realize I have taken in a too big a challenge, I don't even consider quitting playing, I just quit the piece for now
While I don't have much "finished" repertoire, I have gained a lot of skills that are transferable to learning pieces of many kinds. Those skills seem to stick even when the pieces they came from are gone. So works for me so far.
Oh, one more thing...Don't try my way without a proper teacher... Trying to do what I do without objective, honest and critical feedback is just not feasible.
Topic: Grade 5 pianist studying the WTC