Well, that's what the DEA says, but frankly, there's every reason to suspect their motives and believe that they're exaggerating that. ("Opiod epidemic" hysteria and fighting to protect people from reacteational pharmacology and medically ridiculous claims about first responders getting sick from performing CPR on "heroin" ODs.) Once upon a time, they told us that one puff of marijuana caused addiction, psychosis and incurable insanity, after all. (Ever see Reefer Madness? It's hilarious in a pathetic kind of way.)
It's definitely very potent, just saying it's probably not as dangerous as the drug-warrior, ban-all-fun-things jackasses say.
According to the FDA, LD50s of IV fentanyl are 3mg/kg in rats, 1mg/kg in cats, 14mg/kg in dogs and 0.03mg/kg in monkeys. That would seem to put the LD50 closer to a few mgs, and a near certain fatal dose at several mgs.
hxxps://www.accessdata.fda.gov/drugsatfda_docs/label/2013/016619s034lbl.pdf
I'm not a recreational "heroin" user, but I've known some, and a tenth of a gram of "heroin" is a standard single use amount. That's .1 grams of powder, which is mostly filler material. So this is probably a case of someone selling a standard single use of "heroin" that's basically all filler, with a tiny amount of fentanyl mixed in to make the powder good for a single high, same as it was back when "heroin" was actually heroin mixed with a bunch of filler powder.