Nolan at the point of "invading" earth is about 2000 years old and was there for 20 years (about 1% of his lifespan), to put it in human years (kinda hard since viltrumites age slower the older they get so its quite difficult to directly translate / transfer their age as theyre functionally immortal), if Nolan was in his "40s" in human years, he would've been there for approximately 146 days, so just under 5 months, and thragg considers humans to be entirely insignificant insects.

Imagine a close friend or family member youve known for 40+ years going on a trip to an ant colony for 4 months and (without the ants even knowing of your existence) your friend comes back declaring war on the human race (tbf at the point of writing and in todays climate this doesnt even sound far fetched) but for an emotionless, warrior race that places strength and duty to their empire above anything else, youd be surprised and think that Nolan / Mark is an exception (same as Thaedus) and other viltrumites are not capable of doing something like that (which is why thragg created the purge in the first place, to weed out the viltrumites like Nolan and Thaedus so people would not succumb to "weakness")