Zelenskyy's political party is essentially a PR firm: he became famous by starring in the _Servant of the People_ TV show playing the Ukrainian president, and then after the show ended the production company Kvartal 95 [1] became the Servant of the People party [2]. I bet this is a big reason they're so amazing at communications - the party was literally born out of a hit TV studio!
No question Zelenskyy and his team have been killing it (uh, so to speak). Outside help or no—if I had to put money on it, I'd bet on "yes, world-class PR and/or marketing firms were engaged, probably before the most recent invasion started"—he and his team have been incredible from hour 1 day 1.