Thing is, that "highly" is a bit exaggerated, meaning "it's toxic alright but lethal in only some cases" and "people" refers mostly to children since they are more susceptible than adults.
Let's look at the numbers - dry mass of Convallaria maialis contains 0,1 to 0,5% of roughly 40 different cardenolides. Most of them more or less toxic. What you are interested in is Convallatoxin which can range from 5 to 40% of all those cardenolides.
LD50 (mg/kg) for Convallatoxin is as follows:
3.65 (cats, oral)
0.18 (cats, intraperitoneal)
0.185 (guinea pig, intravenous)
0.2 (guinea pig, intraperitoneal)
1 (mice, intravenous)
10 (mice, intraperitoneal)
0.09 (monkeys, intravenous)
Also, keep in mind that symptoms include heavy vomiting and diarrhea so your body will attempt to expel most of the poison, one way or another.
Given that and quite wide range how much toxin specific plant can contain. while it is possible to kill yourself with it. i wouldn't recommend it due to high unpredictability and nasty side effect you'll get should your plan fail.