Hi, this is Dylan, a co-founder and engineer. I'm sorry we didn't get back to you sooner. It's tough to respond to every single comment on a Hacker News thread and we've been quite busy with launch. Hope you can sympathize.
Your code runs in its own Node execution environment and VM, completely isolated from other users' code.
Happy to answer any other specific questions you have! Feel free to reach me at dylan [at] pipedream [dot] com .