The phrase or similar ones is/was used in various ways to assert somewhat underdetermined notion of freedom or even of total ownership; NPR:
"Probably it is true that most American college students, for example, who chant 'from the river to the sea' do not mean to evoke this idea of ethnic cleansing, do not mean to call for the erasure of Israel or the destruction of all Jews in that land," said Julie Rayman, managing director of policy and political affairs for the American Jewish Committee.
"But unfortunately they are echoing that exact trope," she added. [...]
In fact, a lot depends on context. The Likud Party of Israeli Prime Minister Benjamin Netanyahu said in its original party platform in 1977 that "between the Sea and the Jordan there will only be Israeli sovereignty."
Although more recently it seems it's been used more by Hamas than by Likud e.g.:
“Palestine is ours from the river to the sea and from the south to the north,” Khaled Mashaal, the group’s former leader, said that year in a speech in Gaza celebrating the 25th anniversary of the founding of Hamas. “There will be no concession on any inch of the land.”
Something like Pepe the Frog, I guess.
And legal-political opinion on it also appears somewhat divided:
Berlin’s government has criminalized the slogan, alongside “Death to the Jews,” Israel’s i24 News reported. [...]
British Home Secretary Suella Braverman said in a letter to police officials that they should consider whether the slogan “should be understood as an expression of a violent desire to see Israel erased from the world, and whether its use in certain contexts may amount to a racially aggravated section 5 public order offence,” the Guardian reported. Braverman was replaced following uproar over her comments, which included an accusation that British police were too sympathetic toward pro-Palestinian supporters.
Last month, a Dutch court ruled that “from the river to the sea” was protected speech, after an activist had been reported to the police for inciting violence against Jews after he uttered the slogan at a protest this summer, before the current war. The judge ruled that the phrase related to the state of Israel and not Jewish people more broadly, according to the European Legal Support Center, which assisted the defendant.
FWTW, a similar expression has been used at least once by B'Tselem, but they don't mean to assert ownership, but rather complain about lack of freedoms:
“It is one regime between the Jordan River and the Mediterranean Sea, and we must look at the full picture and see it for what it is: apartheid,” the group’s executive director, Hagai El-Ad, said in a statement.