The PSL split from a party that was, many decades ago, a Trot party. But the PSL itself has never been a Trot party (split memes notwithstanding), and that history is pretty irrelevant to what the party is today.

The PSL doesn't really bother to have "a line" on Trotsky or Stalin. Rather, it correctly eschews that "great person" framing of history and instead analyses the historical material conditions of the Soviet Union etc. It's certainly not "anti-Stalinist" like Trots always are; nobody is going to look at you strangely for speaking positively of Stalin.