Twitter definitely has a moderation strategy and actively is following it. It doesn’t mean it’s 100% but that it’s a good faith attempt to do so. Trying to argue Twitter systematically allows death threats to some people while banning others is an extraordinary claim that you should back up with proof (or remove).
It is because Twitter is an older platform and obviously has an active moderation team. It seems either Parler is unwilling or overwhelmed to clean up shit.