I'll take a stab at this but I also think this is really a two-parter query. One part I can answer pretty well, the other I think is more speculation on my part (and actually quite hard to find sources about, it seems).

The first part of the question (the one I can answer) is whether the irregular fighting that played a role in the Peninsular War was so jaw-droppingly revolutionary that it defined a whole new understanding of how wars could be fought. It was not. Irregular warfare, partisans, etc were not revolutionary concepts at the time of the Napoleonic Wars, though it definitely was not as well-studied and defined as we view it now. What we would now consider to be "guerilla warfare" was a common enough strategy employed from ancient history onwards whenever one side of a conflict found themselves engaged in an asymmetric struggle. Max Boot, Senior Fellow at the Council on Foreign Relations and author of "Invisible Armies: An Epic History of Guerrilla Warfare from Ancient Times to the Present" argues that guerilla warfare is essentially as old as human conflict. Specifically, he notes that the United Kingdom only came to be united after the English successfully suppressed Welsh, Scottish, and Irish guerrillas during the medieval period. Boot also cites Thucydides account of the Aetolian highlanders engaging in what amounted to guerilla warfare to confound heavily-armored and more regimented Athenian hoplites. Martin Walker, Senior Scholar at the Wilson Center, cites an example more pertinent to the Peninsular War, that of Francis Marion, the infamous "Swamp Fox" responsible for harrying British soldiers and escaped slaves joining the British in South Carolina during the Revolutionary War. Walker notes that Marion himself learned his guerilla tactics from his time spent fighting the Cherokees, which further highlights European experiences with irregular warfare prior to the Peninsular War, especially in the colonial context. Even more pertinent to the French experience in Spain perhaps was the uprising in the Vendee in 1793, which would have seen a fair amount of the type of scattered, irregular warfare that the French were to experience in Spain and Portugal. As such, the idea that small, ill-equipped bands of individuals with local knowledge and support could harass, disrupt, and otherwise make life difficult for full-scale armies would not have been a shocking revelation in the Napoleonic era.

Which leads to the second question, and one that I am less equipped to answer. What gave rise to the common usage of the term guerilla warfare? Obviously, the term originates in the Peninsular War. But after the conclusion of the conflict, irregular warfare or partisan warfare remained the more constant definition in military parlance to describe such actions, especially outside Spanish-speaking countries. For example, during the American Civil War, guerilla fighters were referred to as "irregulars" or "partisans" or "bushwhackers" or "raiders" but rarely, if ever, as guerillas. In the Boer Wars, guerillas were referred to as "commandos", owing to the "commando system" the Dutch East India Company put in place centuries before the conflict. So when did "guerilla warfare" start to become a commonly accepted term to describe, well, guerilla warfare? I would speculate that the usage of the term in the writings of first, Vladimir Lenin, in his 1906 work "Guerilla Warfare", and then Mao in his 1937 text "On Guerilla Warfare" probably started to push the term to the forefront of public consciousness. Che Guevera wrote his famous handbook/treatise, "Guerilla Warfare" in 1961, which is probably what firmly cemented the term together with the concept in public understanding when combined with the increasing number and prominence of guerilla warfare campaigns in the post-WWII world. Again though, that's my speculation, and hopefully there's somewhere out there who can better answer that aspect of this question for you.

Sources

Max Boot "The Guerilla Myth" Wall Street Journal https://www.wsj.com/articles/SB10001424127887323596204578243702404190338

Martin Walker "Guerilla Warfare's Epic History" The Wilson Quarterlyhttps://www.wilsonquarterly.com/quarterly/winter-2013-is-democracy-worth-it/guerilla-warfares-epic-history/

Kara A. Kozikowski "Guerilla Warfare: Hometown Heroes and Villains" Battlefields.orghttps://www.wilsonquarterly.com/quarterly/winter-2013-is-democracy-worth-it/guerilla-warfares-epic-history/

Max Boot "Invisible Armies: An Epic History of Guerrilla Warfare from Ancient Times to the Present"