Desjardin auto insurance is the former State Farm, and they only wanted to insure factory-stock vehicles.
As for aftermarket parts ... Some of it is "fear of the unknown", some things really do present additional risks, some of it is being unable to separate properly designed modifications from hack jobs.
There are certain aftermarket modifications that legitimately have additional risks. Truck "lift kits" are nothing but bad in terms of what insurance companies care about. Increased roll-over propensity in all situations (and roll-overs are very, very bad for occupants), bumper and crash structures are now at incompatible heights with most other vehicles on the road, vehicle stability can be significantly impaired in ways over and above roll-over. Not all of them are structurally safe, either.
Insurance companies don't like anything that will make a vehicle go faster. They think they can estimate the risk of the stock vehicle. Modifying it means they think they can't. Even for something that doesn't make a real difference (like the exhaust system that the original story was about).
When my previous car got a broken front spring and a strut with no damping on stock suspension that was never known for being all that great anyhow, I replaced it with a complete Bilstein spring and damper kit that was properly engineered for the vehicle and set it to a non-stupid ride height. (Huge improvement over stock.) World of difference between that, and someone who chopped stock springs and ovalled out suspension attachment holes to get more camber, nevermind that the car is riding on the bump stops and the suspension is incapable of absorbing actual bumps.
Insurance-related funny situation. Same car. I got tapped from behind on the 401 in rush hour, and it broke a piece of the exhaust system. OEM replacement was $1300 and was going to take three weeks to get. A local performance exhaust place had an aftermarket replacement in stock and it was $500 if I remember right. I presented this situation to the adjuster. Insurance paid me to put an aftermarket exhaust on my car! (no difference in performance ... but it had the result of giving me a lifetime warranty on a certain flex-pipe that was notorious for breaking on those cars)