I don’t agree at all that it’s implied that these people never came across other humans.
It’s fairly clear, from the tales of the Destroyer/scavengers that Uhura and the others recount, that these humans do not care who they are attacking or killing to gain resources and technology.
They are known to attack and raid colonies and destroy entire planets. While some were populated by alien species, others were human colonies based on the reports.
They surely knew from previous seizures of ships that some were human crewed, by the bodies if not through language.
They had become a voracious pirate culture.
It’s not obvious that communication could have turned them away from destroying the populated planet that they were on course for.
The outcome of Kirk’s decision is that the Federation didn’t get the opportunity to try to communicate with them before destroying them as a last resort.
Discovery became increasingly hopeful and positive as it went on.
Worth watching through season two at least if you haven’t already done so.
I liked seasons three and five a lot.
Season four has a really great classic Trek premise but the constraints of the COVID protocols led to some dialogue that’s over drawn out (Picard season two suffers the same).