I think misinformation is not really that problematic as the truth is valuable to most people and we seek it out all on our own. Its self correcting in an open society.
Note that he specifically says in that passage that he’s not advocating the suppression of all intolerant speech. Thus even if some speech were intolerant, that by itself would be insufficient for suppression.
---
Misinformation is problematic to the extent that it endangers the open society’s very existence, and with it its ability to self-correct.
Many leftist would meet this criteria just as strongly ala "My <x> is not up for debate." or other statements conflating ones existence with ones identity with the same result: No discussion (outside a select, tautological group of gatekeepers) allowed.
"Less well known [than Plato's paradoxes of freedom and of democracy] is the paradox of tolerance: unlimited tolerance must lead to the disappearance of tolerance. If we extend unlimited tolerance even to those who are intolerant, if we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them.—In this formulation, I do not imply, for instance, that we should always suppress the utterance of intolerant philosophies; as long as we can counter them by rational argument and keep them in check by public opinion, suppression would certainly be most unwise. But we should claim the right to suppress them if necessary even by force…" (The Open Society and Its Enemies, ch. 7, note 4)
What's clear from this quotation is that Popper advocated neither unlimited suppression of all wrongthink (as the gleeful Nazi punchers sometimes misrepresent him, and indeed advocate themselves), nor the opposite extreme of unlimited tolerance of any political speech so long as no violence has yet been employed (as the more libertarian among us sometimes misrepresent him).
And that's the key to how I'd respond to your point about the "general level of ignorance". I think you may be right that the general level of ignorance has not worsened. It is in any case quite difficult to measure such a thing. But the prevalence of misinformation specifically about foundational matters of democratic governance, such as whether one can even trust the elections themselves, being promoted by actors with open disdain for open society, and a clear intent to end it, is the precise sort of situation I believe Popper had in mind with this passage.