It is absolutely magnificent to see a electric truck full of sand driving through the street, here is a (sorry, verry bad quality) video of a BYD one I caught: https://youtube.com/shorts/B0akomAQgkM?si=B1JEkKrTk6w7q5Bq
Though I love the sound of a straight cut gear, I suspect they'll want to work on quieting that drivetrain a bit. Thankfully there is a lot of prior art.
It likely uses a transmission as a tradeoff between torque and RPM of the electric motor or because it's a simpler task to retrofit an existing truck driveline with an electric motor bolted onto it.
These are so noisy because of the use of straight cut gears, which you choose when preferring strength over noise and comfort. Most regular cars use helical gears which reduces the noise due to continuously meshing between gear teeth, whereas straight cut gears are like a paddle wheel, so the teeth are smacking into eachother head on each time they engage. That is also why they clunk when getting on and off throttle.
We have some similar trucks running around Australia at the moment and the sound is the same, I'm also familiar with the sound from motorsport!
Food for thought though, trucks are much much heavier loaded than cars. 100-150+ tonne vs like 1.5-4 tonnes for a car. Cars are trivial by comparison.
Perhaps the motor they have available doesn't have enough torque for it without gearing down.
This is true of cars too - the single-speed transmission in a Tesla is a 9:1 reduction gear, for example.