I don't think most SaaS companies follow Lean Startup principles. Most of the ones I've had to integrate with have had horrid infrastructures and extremely brittle interfaces. It's a delight for me to actually have properly-maintained documentation available on the web and not an out of date Word document that I have to go back and forth with one of their engineers to understand. Non-standard format implementations are the norm and not the exception.

The general gist I got is that sales guys pluck mediocre, but hard-working engineers from the corporate ranks and build businesses around them. The whole thing is kept together with long hours and elbow grease when it frequently and inevitably breaks.