Its just a letter complaining about a product being pirated. It only shocked people because the idea of end-consumers paying for software was novel then.

The point is that it demonstrates the core ideas which Bill Gates put in action in Microsoft. I.e. aggressive IP methodology.

Piracy is not the same as "open technologies" in any way. In fact, the latter prevents the former (if I settle on GIMP instead of pirating Photoshop...)

If you read it carefully it's not just about piracy. It's against sharing (which in open source happens without any piracy). His basic idea - sharing prevents good software from being written. I.e. only closed paid model can be incentive to create good software. It's the cornerstone idea of Microsoft and is very in line with their historic hatred towards open technologies which they always saw as a threat (and obviously not because of piracy). It also is in line with their aggressive approach to enforcing patents and copyrights.