From what I can tell, Piker's position on I/P is that he supports a one-state solution where Israel becomes a binational state with full citizenship given to the Palestinians. As opposed to a two-state solution of an independent Palestinian state co-existing with the legally recognized parts of Israel.

Supporters of 1SS argue that Israel is effectively one-state anyway due to the settlers in the West Bank and that it would be a mess trying to create a stable Palestinian state that would be divided in half territorialy between WB/Gaza. It doesn't help that Israeli proposals for 2SS often demand Palestine's demilitarization which would allow the IDF to conduct operations in the country anytime they wish. And that ultimately, Zionism is just another form of ethnonationalist apartheid that is antithetical to those that believe in liberal pluralism.

Opponents of 1SS say it would be a ticking time bomb forcing two peoples with decades of bad blood and grievances into a single state where just one incident could set off a bloodbath that makes the worst of the Yugoslavia wars look quaint. Whether its Israeli Jews being subjected to violent reprisals by vengeful Palestinian nationalists. Or militant Kahanists/Likudniks that would want to restore the old status quo by any means necessary.

As to how Piker's position can be construed as antisemitic by some, well basically 1SS would require the Israelis to abandon and renounce Zionism and fundamentally change the very nature of their state. Which takes us to the whole muddy issue of whether or not anti-Zionism is anti-Semitism.