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.

Let's look at it in a different way. Are these rats doctors? Well, definition 4 of "Doctor" from Merriam Webster's says ": a person who restores, repairs, or fine-tunes things." These rats are certainly restoring the fields by identifying the mines for removal. We can just anthropomorphize the rats to get by the "person" part of that definition and conclude that the rats are, in fact, doctors.

Are the rats lawyers? Again, from the same dictionary the first definition for "lawyer" says "one whose profession is to conduct lawsuits for clients or to advise as to legal rights and obligations in other matters". That definition is of the form "X or Y" and Y would be "profession to advise as to legal rights and obligations in other matters." The rats, of course, aren't removing the mines themselves. They simply locate the mines and inform their human employers who then defuse and remove the mines. We could say the rats are advising the humans. If the humans running this operation come to know about a mine they clearly have an obligation to remove or designate the mine for removal - knowingly leaving a land mine in a field is liability. So, these rats, in their professional capacity, are advising the humans with regards to their legal obligations. The rats are lawyers!

Are the rats scientists? Merriam-Webster tells us that scientists include "scientific investigators". The rats are clearly investigators. Are they scientific? The definition for "scientific" includes "practicing or using thorough or systematic methods". Well, the rats search an entire field. The article tells us that the rats scour fields so thoroughly that the human operators are willing to play soccer on the fields afterwards. That sounds pretty thorough to me, so we can call the rats scientific investigators, or scientists too.

I hope you get my point. Using this "sophisticated" conception of language, where you look at dictionary definitions and suggest that one interpretation of some clause of a definition could technically apply so therefore a word is apt to describe something leads to absurd conclusions. Not only are our rats heroic, they are heroic lawyer scientist doctor rats! And more!

The truth is that dictionary definitions are guides to help us understand language. An articulate person will strive to find words that fit well, not words that, by some torturous and philosophical contortion could technically, arguably, maybe not be entirely wrong if you ignore a couple words. "Hero" fits these rats the same way doctor, lawyer, scientist, engineer, and so on fits the rats - which is to say, not well.