I don't think it's the format. Forums generally get toxic when they're too big. The negative influence of a toxic user is much greater than the positive influence of a non-toxic user. The bigger the user base the more toxic users. Eventually it gets to a critical mass where you're seeing enough toxic replies to make the whole platform seem toxic.

Reddit is 18 years old. Lots of time to attract toxic users. I wasn't on Reddit from the start, but people have said Reddit didn't suffer toxicity until after it was around 10 years old. Lemmy is four years old now so it will be a while. Though Lemmy may attract a smaller less toxic crowd and avoid toxicity indefinitely.

I don't have a high opinion of community at Stack Overflow as it started out elitist by nature of its policies and rules. Yeah that's going to breed toxicity right out of the gate. I have to admit Stack Overflow has been a really good resource for technical information at times, but its community is harsh. As much as I've used it to find good technical information, I've never made an account there or had any desire to post there.