It is very, very difficult to poison a rabbit that is used to greens, they evolved to deal with everything botany throws at them. Either by stomaching it, or by deciding after a test nibble that they don't like it, or just in small amounts. There are many shades of gray regarding toxiticy, if they have free choice and practice they know how much they can stomach, or even use it as medication.
One can get pretty paranoid when reading too much on the internet.
Those lists on the internet are well meant, but often everything that is toxic to any animal is on there, like, how did horses manage to survive so far? And they don't take into account that rabbits can deal with quite a lot of that stuff and can taste what is not good for them.
The test nibble he did isn't going to harm him, I wouldn't put that plant on his daily diet though.