Based on the article, the inhibitor chemicals _are_ the passive protection system, they just can't be perfect because too much of that stuff ruins the purpose for having the chemical in the first place.
> The use of high levels of inhibitor can cause the monomer system temperature to far exceed the onset temperature of thermal polymerization under external heating. Once the inhibitor is exhausted, the thermal runaway reaction proceeds at an elevated temperature with a substantial reaction rate and very little reactant/monomer consumption.
Source: This fascinating paper linked to by fuzzfactor in yesterday's (edit: 3 days ago, lol) thread:
https://iomosaic.com/docs/default-source/papers/polymerizati...
The comment: