Lambs quarters are fine in moderation; the issues with oxalic acid are not scientifically proven and they (I'm guessing the Tortoise Table) aren't suggesting avoiding specifically due to hydrogen cyanide. I don't I've ever seen anything here about hydrogen cyanide either; it "sounds" bad, not sure that it is.
Plenty of people feed it, at least in moderation. My Russian isn't a fan but I still offer it on occasion. Same goes for amaranth which is in the same plant family. She'll only take a few bites a year but it never hurts to try.
As always, only you know what is best for your tortoise. No reason to feed anything that makes you uncomfortable.