This is true, however:
Burying is not the same as being thrown in the ocean, or ripped into little pieces and thrown on the ground.
Burying would likely keep plastic in the immediate vicinity, and we have another tool to add. Bacteria.
There are bacteria which eat plastic. If we could find some anaerobic ones, burying might work quite well.
Of course I guess that leaves the carbon free again, unless...
We find or engineer bacteria which produce a great precursor, to make plastic!
It would be a strange outcome, if plastic became fully renewable.