The arbitration panel, who ruled for Mexico, says that the US has the right to set truck standards, whether that be driver standards, safety standards, insurance standards, enviro standards, or any standard, as long as the standard is the same for Mexican trucks as it is for US or Canadian trucks.
And, if a Mexican truck pulls up the border, and doesn't meet the standards, then the US can legally deny that Mexican truck entry.
But that is not what happened. The US took the position that, collectively, Mexican trucks are not as likely to meet US standards as, collectively, US trucks.
So, even if an individual Mexican truck, or many, many individual Mexican trucks do meet US standards, they are not allowed entry.