What vallaslin meant during the time of Arlathan doesn't reflect what it's come to mean now for the Dalish, which is also what Lavellan can say to Solas.
Yeah, I don't know why people keep saying this. It has meant something to them for generations, its original meaning is irrelevant unless an individual elf decides they take exception to it. There's plenty of things in human culture that meant one thing long ago, but mean something different now. It's good to know, for personal reasons, what it originally stood for, yes. And for those elves who could be upset about it, it is something they definitely deserve to know. Some will hate it, and some will choose to embrace it for what it is, an aspect of cultural heritage that has been re-appropriated to be something different than what it meant a thousand years ago.
And who's to say that when the elves fell, way back then, that they didn't deliberately re-use those tattoos despite knowing their origins? Maybe they knew what they started as but said f*ck it anyway, it's going to be this now, and taught that to future generations, in which it became distilled into what it is now, its original meaning lost?
My elves will not give up their tattoos, given the option. But they won't look down upon anyone who does (assuming that is ever an option to anyone not in the romance). It's personal. As a person with tattoos that I got for specific reasons, I can totally relate to it (though clearly mine don't have an origin like the elf tattoos). If I found out my tattoos' origins were different than what I thought, that doesn't change what they mean now in the present. (One's from a video game though, so no chance of that happening.. heh.)