We are talking about the CoC so we have to use the clear definition of that word. Based on the Oxford Dictionary, the definition of the adverb “intentionally” is
[to do sth.] Deliberately; on purpose.
Other dictionaries like Dictionary.com and Merriam Webster define “intentionally” pretty similar:
done with intention or on purpose; intended:
of or relating to intention or purpose.
done by intention or design
We can talk about other definitions of “intentionally” but due to the fact that this definition exists, it applies to the CoC as well.
I can’t do it directly. I’ve found an Interview in which Jeff stated that One-Tricking isn’t a bannable offense as long you play your best. However, he also mentioned that discussions on one-tricking mostly are focused on individuals who got actioned or banned but with the notification that other factors had an impact on the penalty.
There are also some threads on the EU-Forum about players who complained about getting banned for one-tricking:
Soon banned for one tricking, what to do?
New TORB account banned after 3 games
Blizzard banning people for playing Torb
How blizzard handles feedback
They might be completely anecdotal and some show other factors for their penalty like non-communication - others don’t.
It does show that there is a blurry line between one ticking being a bannable offense or not, even if Jeff said that one-tricking alone isn’t a bannable offense.
Add the fact that you play a hero terribly - you are learning after all - and that line is even blurrier.
Since the question of whether learning a new hero on your main account might be considered a bannable offense is highly debatable, due to the fact that multiple facts clash onto eachother, let’s focus on another question:
Why should learning heroes on your main account be a better Idea than doing it on an alt-account?