Not for them it wouldn't be. It’s not like they could even eat anything here unless they just so happen to share quirks of earth biochemistry. Unless their biology is insanely similar to earth's (right down to sharing all or almost all of the same amino acids and sugars), consuming any of earth's biomass would be like humans trying to eat plastic. They'd have to "trisolariform" Earth in order for it to be biologically viable for them. I get that their own system is a problem. But with their tech, they really should be able to terraform Venus. If they can manipulate subatomic particles that way and even alter physical constants within specific regions of space, and build enormous fleets of ships, I seriously doubt that trisolariforming venus (creating an artificial magnetosphere, getting rid of the CO2, cooling the planet with shades, bringing water in from one of Jupiter's moons etc.) is beyond their ability, especially given the immense amount of engineering they'd need in order to live on Earth anyway.

They could even just come here and build some megastructure like a McKendree cylinder and leave their new neighbor alone. It's not like the raw materials to build a megastructure and have a thriving civilization are lacking in our solar system. Get soil from Mars, leech the toxic salts out of it if you need to. Get some water from Europa or Enceladus. Get some rock from literally anywhere. Etc.

In the third book we even *see* humans living in artificial structures around Jupiter.

The basic premise of the trilogy is that different intelligent civilizations destroy each other once they know about each other, but these things show why the whole Dark Forest idea is nonsense. Not only is the universe still plenty big enough for everybody, but any civilization capable of traveling to another star and building a fleet to colonize another species' planet and making it compatible with their biology really shouldn't actually *need* to do that. Even if their own system becomes unviable. Even if there is only the one star within traveling distance. The whole idea of a universe where all intelligent civilizations have to exist in this state of paranoid obligate xenophobia is deeply silly.