You see US subs roaming the world, though. Why can the US track Chinese subs in the Gulf of Mexico but China can't track US subs, in, for example, the East China Sea?
I'm not sure if you are being deliberately obtuse or not.
Why do you think China can't track US subs in the East China Sea?
Furthermore, how on earth does that even relate to his previous point?
Do you recognize what "being able to track a submarine for its entire duration" means in terms of physical assets and basing?
It means that if I want to track your submarine for its entire global distance deterrence mission, I can track it from right outside of your doorstep because I have a permanently forward based presence in your hemisphere of the world, and as your submarine leaves port, my prepositioned assets in the region can give me a much better chance at tracking your submarines even if they are very stealthy and quiet.
Those permanently forward prepositioned assets, as well as the rest of my air and naval bases around the world, will allow me to have a good chance of achieving constant monitoring of your submarine as it goes from:
"Base -> transit -> mission area"
The US has a global military presence around the world, including in the western pacific close on China's doorstep, allowing it to allocate assets to do the tracking of Chinese submarines in the "base -> transit -> mission area" cycle and it allows the US to achieve handover between those stages in a way that China cannot do.
Of course, developing more stealthy submarines as well as supporting assets to try and interfere with US peacetime ASW and tracking operations can be done -- but it doesn't change the fact that the US has a massive network of bases on China's doorstep.
Meanwhile, China
doesn't have a massive network of bases within 500nmi of the US coast, and doesn't have a massive network of bases spanning the entire pacific and atlantic either.
That means they are only able to try to detect and track US submarines once US submarines have reached the "mission area" (i.e.: western pacific, in China's case).
Now, Chinese ASW has improved in leaps and bounds over the years, but that doesn't change the geographical reality of the basing of physical assets and infrastructure.