Wasn't Earth supposed to be technologically superior to the Dilgar?
I mean, back then, AFAIK all Earth had was those old Hyperion-class destroyers, but IIRC even these were sufficient to defeat the Dilgar.
(Noncanon musings)
80 billion Dilgar standing on a planet ripe for slaughter by their sun going nova.
The actual Dilgar top tier military, under command of the politicians, generals and billionaires, pick an unoccupied star system and take a hundred million useful people with them, because that's how many ships they have, and that's how many people they can move in a year for 60 light years, to build Dilgar II.
That leaves 79 billion Dilgar abandoned with only civilian ships and broken down military vessels that no one wants, unloved and left to die. Which is the second wave who have two missions, find enough star systems for 79 billion Dilgar to inhabit comfortably, with infrastructure, because all the people that could design or build a sewer for 30 million people are already gone in the first wave. Mission 2 is kill the first wave, because "What a pack of ####s, what a pack of amazing ###s" but because they are not the best people with only the ships that no one wanted in the first wave, they all die because they suck and their tools suck.
Wave three is the telephone sanitizers, and keyboard cleaners, and they are just glad to have made it off world before Dilgar 1 went boom, by which point every civilized society in space has kill orders for every Dilgar ship and every Dilgar face, so they just get murdered on principle and poor luck, until there are none left.
There is no wave 4. 70 billion dilgar probably die when the sun went nova.