muslims are technically right when they say that jesus was a muslim. originally jesus was a jewish sage that promised to be the long-awaited messiah. He considered himself a human being, and in the gospels he distinguishes himself as a separate entity from god, even telling people to worship god instead of him. He stresses on the fact that his teachings should not be taken to gentiles. He never talks about taking away humanity's sin nor the idea of a trinity nor does he nullify the torah law, instead telling his disciples to strictly follow the law.
All those ideas come from paul of tarsus who never met jesus yet changed all his ideas just to convert gentiles. Jesus's disciples and immediate family members like james his brother attacked paul for teaching false things and called him an apostate. This was the real christianity, what jesus preached and it survived for a long time as ebionite christianity.https://en.wikipedia.org/wiki/Ebionites
The muslims are right in saying christian scriptures are corrupted since paul changed them. A muslim means one who submits to god, which jesus a muslim even if we just read the gospels.
It seems that this criticism of paul is not recent, but its been known for a long time, since the quran talks about again and again. Islam, inspired by ebionites was a philosophy created to solve this issue, but seems to have created an even worse bastardization that made it worse.
All in all, when muslims say jesus was a muslim, you cant deny that because they are technically right.