“Leftist” is not a helpful label here; its meaning changes internationally and personally. It was always vaguely defined and just became more vague and misused for the past two centuries.
This is an issue because:
It leads to unresolvable persistent conflicts over what is leftist and what isn’t, and therefore who is welcome here and who isn’t.
The admins’ definition appears to be different from some very common definitions. In the post ‘What is lemmy.ml?’, they imply that a ‘liberal instance’ is ‘something that [lemmy.ml] is not’. This will at best lead to repeated rejection of people who consider themselves ‘leftist’ but whom many users do not (an annoying and useless exercise for everyone involved), or at worse subversion by people who think they’ve found home and need to defend it against ‘extremists’.
Maybe consider ‘anti-capitalist’ or ‘socialist’ as less ambiguous terms, assuming that is what you meant. This will avoid users who identify as leftists mistakenly signing up and defending the place against those it is explicitly made for.
As a demonstration of the wide range of political positions reasonably considered by people to be ‘leftist’, here is the Wikipedia article for ‘Leftism’. Common definitions include ‘‘pro-egalitarianism’’, ‘‘liberalism’’ and various ‘progressive’ social rights movements.