What actually erodes
Technique learned over years of training does not disappear over a layoff of months, but the fine timing that comes from live sparring and fight-paced repetition can dull noticeably, along with a fighter's calibration for how hard and how often they can push a five-round pace. These are real, measurable effects, distinct from the vaguer claim that a fighter simply 'loses it' by sitting out.
The injury underneath the layoff
Long layoffs are frequently caused by injuries that have not fully resolved even once a fighter is medically cleared to compete, and a fighter returning from a serious knee or shoulder injury may be dealing with compensating movement patterns that have nothing to do with time away from competition itself. Attributing a poor performance to rust alone can obscure the more specific, physical explanation underneath it.
A convenient explanation
Because 'ring rust' is easy to say and hard to disprove, it gets applied after losses regardless of whether the layoff was actually the deciding factor, when a straightforward stylistic mismatch or a simply better opponent might explain the result just as well. Treating ring rust as a spectrum of specific, checkable effects rather than a single catch-all verdict gives a more honest account of why a returning fighter performed the way they did.

