Non-round stuff in space of a sufficient mass does tend to shift back to being round over the course of millions of years, so absence of non-roundness is not proof that parts of it were not ripped away at some point or other. Uranus is a gas giant (actually some call it an ice giant) so there's no way it would ever take a non-round shape for more than a few instants).

What you call "space agencies" is in this case Durham University's department of Computational Cosmology; if you read the article, there is no need to blindly believe anything: Uranus had been observed having strange behaviour compared to other planets in the solar system, and the researchers at Durham just built a computational model that shows how you can "derive" the observable behaviour of Uranus by starting with a "healthy" planet and simulating an impact with a massive object. That's it.

Is it the Holy Gospel? No. But it's a simple explanation which does not violate any law of physics and which does not necessitate further hypotheses being made. Is it the truth? Who knows, maybe in the future we will disprove this theory in a conclusive way and a new hypothesis will take its place: that's how science works, no belief needed.