They can be both. I wouldn't recommend eating any venomous animal as their venom may poison you. The venom may be digestible, but get a tiny scratch from one of those fangs and you will not be happy.

Can be, but iirc, there is only one snake that is in fact poisonous (to eat). Most snake venom is complex, unstable molecules that your stomach acids would destroy pretty quickly.

I happen to have an Oxford English Dictionary next to me so I took a look. Definition 1 for poisonous is "Containing or of the nature of poison; having the quality of property of poison; venomous".

1665 ... poisonous viper ...

The OED is listing common usage, while the biological jargon is specifically drawing a distinction in the way the poison is delivered by an animal.

Yes; the article itself does refer to them as venomous but people are quoted as saying poisonous, though maybe the article's use of 'venomous' is a correction that happened after your comment.