Intrusive, maybe, but a dark pattern is 'deceit by design' whereas what you're describing seems to me 'being slightly annoying by design', which is a totally different thing.
Thinking about Exit Intent and how to optimize for it is a fairly well established part of online eCommerce. I think you made the wrong choice and your boss was probably right to be annoyed with you. It's within their prerogative to decide what user experience they offer to their users. And obviously, it's also yours to quit, but if you do so based on a poor understanding of what a dark pattern is, you're just hindering yourself.