Other standards, and web standards in particular, tend to have several open source implementations to guarantee that it's possible to build tools that support it without being the reference implementation. OOXML has no such thing, and the reference implementation is closed source, so it's basically impossible to replicate in any practical sense.
Not fully compatible with PowerPoint, no. That's why I said the PowerPoint format is proprietary; it's impossible to build a working interoperable tool, because either parts of its format remain secret and do not follow the published standard, or the format itself makes references to hidden implementation details.
Having an open source tool that can open files created on PowerPoint without them becoming horribly mangled, or vice versa, is a very realistic concern. Microsoft Office is well known for being extremely difficult to make its file formats portable beyond the very basic layout features.
