Certainly from a mainstream political standpoint he appears to be fairly liberal with some progressive policies. However, the writer is using the term ‘leftists’ to mean socialists or left-wing “radicals” (whatever that means).
His stance of Israel is really what will be the clincher for leftists, as is the case with Harris. On the plus side, they are both taking a softer line in terms of how they discuss the genocide in public, but of course neither of them would ever utter the phrase with relation to the Palestinians – that would be too radical.
Therefore, there’s a lot of doubt as to whether either of them will break from Biden’s policy of continuing to send bombs and military hardware to Israel, as both are apparently very much in the “Israel has the right to defend itself” camp.