If there were some evidence of extreme devotion or admiration, for example maybe the locals celebrate this rat, know its name, call this one for especially challenging fields, erect statues to it, etc - then I'd agree that a specific rat could be heroic by this definition.
As is, the rats seem to be doing a not particularly challenging or dangerous job in exchange for payment. One rat is retiring after a successful career - which is laudable, but not so much so that it qualifies for extreme devotion or admiration.
In general, I like to push back on cheapening words like "hero" so that we can use them for better effect when they actually fit. I also don't think teachers, hospital workers, police, soldiers, etc are heroes though some people sometimes call them such. Individuals in those professions might be, but the general classes are not, nor are bomb defusing rats.
Although the synonyms section cracked me up cause it definitely took me a second
Synonyms “Cuban sandwich, grinder, hoagie (also hoagy), Italian sandwich, po’boy, …”