You had me until thermodynamics. We're obviously not getting every substance in our bloodstream like we are with plastics so I don't think thermodynamics is the issue here.
I think he was referring specifically to entropy in his reference to thermodynamics. Once we extract the source materials from the earth, turn them into plastics and distribute them, there is no cheap way to undo it.
The following sentence was about dispersion in the world and not about the chemical reactions in creating plastic, so I don't think that's what was meant.
Quoting Wikipedia: In statistical mechanics, entropy is an extensive property of a thermodynamic system. It is closely related to the number Ω of microscopic configurations (known as microstates) that are consistent with the macroscopic quantities that characterize the system.
and
The second law of thermodynamics states that the entropy of an isolated system never decreases over time.
Applied here, when plastic breaks into pieces, the number of microstates -- ie, the entropy -- increases.
It's not the same as the ideal gas law, but similar.