If someone steals a replay and submits it as their own, in my opinion that definitely falls under our pre-existing stance on plagiarism. It's probably fine to formalize that into some sort of rule.
However, if someone takes one of your replays and edits it in a significant fashion that improves its capacity to break whatever record it's being submitted for, I find myself less opposed to this; particularly if that replay was already submitted by you for that record. In short, it's a quick and gritty statement of "You were lazy, and you could've improved this in these ways". Ideally, records aren't trivially beaten like that, so please check your replays for easy improvements before submission.
If it hasn't already been submitted for a record, then the requirement for truly significant edits increases, and at the very least the original replay maker should be credited if not contacted, in my opinion.
As for whether or not plagiarism is victimless: that's not a particularly strong argument in this case. The Book of Records ideally keeps track of people who, based on merit, have managed to achieve great things and those same things. If the people they're tracking don't have that merit, then the Book of Records is failing at half of what it's supposed to do; which is to give people credit for trying new and interesting things.
Furthermore, particularly concerned people might be unwilling to post replays and other materials on the basis that others might decide that it's not worth the risk of getting their hard work stolen for some stupid record. Whether or not that actually harms them is irrelevant: it's merely whether or not people decide not to contribute because of it. It's not the potential individual harm that makes plagiarism bad, but the rampant attention whoring it implies and the damage it does to the community (by discouraging content creators from being recognized) and the community's reputation as a whole.