To Zionists, Zionism means that Jews have the right to have a homeland, free of persecution. To non-Zionists, it means that Zionists think that they have the right to a specific area of land (Israel) and that that land is their god-given right, and that they are free to use violence to obtain it. To a secular person, the idea of someone having a "god-given" right to a piece of land is insanity.
"From the river to the sea" has been used to mean "Palestinians will be free everywhere" and also "the Jews that are violently occupying Palestine will be killed from the river to the sea".
I can't speak for specific protestors you encountered, but the majority of people I know that are anti-Israel don't want "all Jews to die" or even any of them. They just want the genocide to stop, for people to stop dying. It's really that simple. Protestors are protesting violence.
I respect that very much, but I think that the problem is exactly that it _isn't_ that simple. If they don't want any of the Jews to die, they should be saying also, alongside "stop the war", how can it be assured that Jews won't die later.
Stopping genocide shouldn't require first providing some solution to Israel's existential anxiety. Israel should simply stop doing a genocide, right now.
Propaganda? I am not very familiar with the details and frankly I don’t really care, but at the two pro-Palestinian rallies that I saw were used "From the river to the sea" slogans, and like all protesters were okay with that.