That is interesting, Einstein was struggling so badly with his tensor equations that without Marcel Grossmann, he could not have solved them.
Einstein himself said, "Do not worry about your problems with mathematics, I assure you mine are far greater. The chance that Einstein was being humble here is too small to think about.
I would say problems with math does not mean he flunked.
One widely held belief about Einstein is that he failed math as a student, an assertion that is made, often accompanied by the phrase as everyone knows, by scores of books and thousands of websites designed to reassure underachieving students.
I really do not know how anyone could feel better flunking just because Einstein supposedly did or did not. Do they live their life by an Einstein barometer? Can you go to an interview and say, "hire me, I failed like Einstein!"
In mathematics, you don't understand things. You just get used to them.
Of course that result can be rigorously obtained, but who cares?
Combinatorics is Algebra and Algebra is Combinatorics.
I think it is just one of those "instantly believable" things that bypass people's common sense.
"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman
I read a great book about Einstein that actually answered most of those questions from that website and showed me a side of him I had never known before. It is called "Einstein's Mistakes"
In my Einstein knowledge I know he wasn't a great "math" person and had the philosophy "I never memorize anything that can be looked up in less than 2 minutes". Marcel, would've undoubtably helped him with those areas he struggled with but another quote, "the secret to creatively is knowing how to hide your sources." So it's not a shock that no formal credit would've been given, only inferred.
I think that knowledge would make others feel better about themselves. Like citing Cindy Crawford has come in second place in a beauty competition. It makes any charge that you aren't as smart as you think you are refutable. Self-delusion dates waaaaaay back.