I was looking at a platform experience engineer job opening and I was wondering how that was different from a qa engineering position? Is it just qa for a platform rather than a product?
As with all positions the title means whatever the hiring company wants it to mean, nothing more.
The term "QA Engineer" might mean "Tester", "Standards Enforcer", "Auditor", "part-time Developer/Analyst", all of these, or none of these. I've seen them all. In my company, the team used to consist of "QA Engineers", until we were purchased by a larger corporation. Now we are "QA Analysts". Our role hasn't changed, just the titles.
The same might be true of "Platform Experience Engineer". It probably means something like "a person who checks to see if our applications conform to our standards for platform experience". Or perhaps not.
Check with the hiring company and with the job posting. Often the stated requirements give you some clues.
The term itself is kind of subjective without more context. In your link you refer to Live Mesh and since someone felt the need to explain (although not very clearly) what a platform experience is it might be an indication how subjective it is.
Wikipedia has a definition to the term experience design which may be a new or more prominent consideration in technology these days.
Depending on what platform experience is there may be some overlap with software testing but it's hard to say for sure.