Self-built cars are usually a completely different process, and also don't involve import.
But, the author of the article also massively overstates the requirements for a legal vehicle under the Low Speed Vehicle rules. The full FMVSS rules are gargantuan, complex, and hard to meet, but the LSV rules really aren't.
They basically need to not go 25mph, meet dimension and weight requirements, and have lights, reflectors, seat belts, and an approved windshield. https://www.law.cornell.edu/cfr/text/49/571.500
The manufacturer then self-attests that the vehicle meets FMVSS, and submits paperwork documenting their VIN format. Unless there are complaints or an investigation, that's pretty much it.
But, the author of the article also massively overstates the requirements for a legal vehicle under the Low Speed Vehicle rules. The full FMVSS rules are gargantuan, complex, and hard to meet, but the LSV rules really aren't.
They basically need to not go 25mph, meet dimension and weight requirements, and have lights, reflectors, seat belts, and an approved windshield. https://www.law.cornell.edu/cfr/text/49/571.500
The manufacturer then self-attests that the vehicle meets FMVSS, and submits paperwork documenting their VIN format. Unless there are complaints or an investigation, that's pretty much it.