And how can any planet under Umiak control be safe from immediate Umiak counterattack?
First we have to assume three things:
1) The Loroi, independent of how they would treat the local population, would likely treat taking habitable planets with their infrastructure as intact as possible as a strategic priority.
2) Defending Umiak would not risk an entire system's defense fleet to burn a world, especially if they know that this particular attack is part of a larger offensive. At best they would send a strike force with the bare minimum of ships (And likely the oldest and least effective ships.) to both glass the world and provide a disposable rear-guard, while what survives of the main force would fall back to a more central system to rally a better defense of their more important core worlds.
3) The main space battle will in all likelihood take place very far from the habitable worlds. (Jump in points would likely be further out from the star's gravity well, while rocky planets would more than likely be much further in. There is also the problem that space battles are very messy, and you don't want megatons of debris floating around in orbit.
(Going to assume that these operations are largely Loroi-led, and that human forces are auxilleries or even just observers, able to communicate with an advise the Loroi forces, and that Loroi would be willing to humor our odd requests so long as they don't interfere with the strategy at large.)
We can assume that Loroi would likely attack a system if they had a force large enough to guarantee success and minimize Loroi casualties, in which case using a detachment of their faster destroyers to intercept and eliminate the Umiak strike-force moving to burn the world would not significantly effect the course of battle. (Especially since, assuming the Umiak have a competent commander, such a force would not be dispatched until they knew that they could not successfully defend the system and have no chance of retaking the planets there in the foreseeable future.) This gives an attacking force plenty of time to orient itself in a way that prevents the Umiak from being able to attack the planet without exposing themselves to attack.
Who cares about the biome? You're not asking for the planet's ecosystems to surrender. It isn't necessary to eradicate all life, and I already described one setup that would destroy any semblance of a civilization at the push of a button, for minimal investment and great gains in security from the Umiak perspective. If they don't do it that way, it still wouldn't take more than a handful of ships to do in a hurry while transiting the system.
We can make a few assumptions here as well. First off, even highly industrialized and urbanized world is going to have a significant portion of it's population outside the urban centers, to provide food and materials for those urban centers. I would imagine if the urban centers are building anything of importance, they would be hardened to some extent, to prevent Loroi raids from causing undue harm if they're able to slip inside the defensive perimeter. (Something that I think is well within the Loroi's capability to do.) Additionally, the Umiak would spread these urban centers out so that a decisive strike against a single area limits the damage done.
While this sort of hardening of infrastructure would be excellent for resisting raids and short attacks from the Loroi aimed at infrastructure, this makes a planet very hard to quickly and effectively depopulate a planet. They might be able to destroy some of their infrastructure ahead of the Loroi, but in order to really burn a planet they'd likely need to know ahead of time that the Loroi would be coming and that this attack would not be able to be efficiently defended. (Strategic/Economic worth of the system vs. the forces required to defend it.)
"But notice how the Human thinks. 'Interesting... how can I use this as a weapon?'" - Arioch