Memes

This person was just as intolerant as a Nazi themselves and didn’t even realize it.

No they weren't, this is what Popper defines as intolerance of the second degree. Taken from the German wiki page because this aspect is better explained there than in the English version (translated with deepl):

In intolerant people, Popper distinguished two categories:

1. intolerance of the first degree: intolerant of a person's customs because they are foreign.

2. intolerance of the second degree: intolerant of a person's customs because they are intolerant and dangerous.

Popper therefore rejected universal tolerance:

"Less well known is the paradox of tolerance: unrestricted tolerance leads with necessity to the disappearance of tolerance. For if we extend unrestricted tolerance even to the intolerant, if we are not prepared to defend a tolerant social order against the onslaughts of intolerance, then the tolerant will be destroyed and tolerance with them."

However, since we as human beings are not capable of knowing the true motives of our counterparts, a fundamental, unsolvable problem now arises: It is difficult for an outsider to distinguish whether a person who expresses intolerance belongs to the first or second degree.

In other words the intolerance against Nazis is justified because they are the ones being intolerant in the first place, and sometimes this is the only way to fight them on that.

I'll grant that this is disregarding your premise of Nazis not existing, but to be quite honest if I may, that's a pretty stupid premise. The Nazis did exist, they were the poster child for intolerance of the first degree, and their ideology is far from being as dead as they are.