It's a good question if you're unfamiliar with this quote.
Once you get to a certain depth of vocabulary, and basic level of skill in writing, it's easy to write a lot of text.
You can sort of "talk around" your central point, adding reams and reams of text, sketching out your point in a crude outline, eventually arriving at a complete picture or just stupefying your audience into submission.
This kind of communication is evidence of "thinking out loud," where you know the vibe of what you're trying to say, but figure out your logic at the same time as you're saying/writing it.
Especially in writing, this would be considered a first draft. If you take the time to think about what you're trying to say then you can often refine/reduce your message to a more respectful length (a shorter one).
I guess you get some thoughtfulness and economy of words kind of for free them. Reminds me of how (I think it was him) Edgar Allan Poe would write with slow-to-dry ink, so he would have to take his time when writing.
I'm guessing the point wasn't to express mere disinterest, but active resentment of the opposing viewpoint: "Not only have I no desire to converse with you, which may be taken as a hesitation to engage with your views, but I believe such a conversation to be utterly worthless because I despise your entire world view" with a dash of "You're a bigot and I want nothing to do with your kind."
That doesn't preclude taking a moment to write such a letter.
If anything, it serves to challenge the pretense of dignified and harmless "opinions" that fascists like to leverage. I'd argue that is much more productive than the way discourse has occasionally evaded calling out the cruel, sadistic, violent, bigoted assholes and enemies of human progress and dignity as just that.
As Russel notes, there is no reasonable discussion to be had with someone so openly endorsing violence beyond reason, whose entire worldview is so diametrically opposed that there is no common ground to found a discussion on in the first place.
Giving fascists the "Eh, just opinions" benefit normalises their hateful views as permissible. For anyone valuing freedom, tolerance, progress and justice, opposing these rhetorics is not just sensible, but even crucial to combat the spread of this ideological cancer.
There can be no peaceful disagreement with an ideology that, given the chance, will suppress all disagreement violently.