The other scaling point is concurrency. You can't (for example) serially add features to a page. Eventually that page will simply take too long to render.
A lot of "dashboard" pages suffer from this problem. If you reach that stage you need to start thinking about decoupling features - using things like messaging.
PS: This is a presentation by Rackspace of course they are going to say write bad software and buy lot's of hardware.
Scale is getting higher throughput by adding resources - Performance is getting higher throughput using the resources you already have.
Scaling is more about a product's ability to easily grow and handle an increasing amounts of work, rather than it's ability to render/dish out pages.
What I meant to say was concurrency is a way of distributing load and getting scalability.... But, yeah, you're right - my example was terrible/inaccurate.