This intrigues me. I founded and actively work on the FlyByWire A32NX, and although the SDK is lacking in some regards, it isn't horribly incomplete (although the documentation is - but much of it is the same as FSX/P3D). Definitely not incomplete enough to delay the development of a study-level airliner by a year or more.
The core systems are likely being written in C++ using the Simconnect API and compiled into WASM. The main roadblocks we've run into are the lack of weather functions (so no weather radar, in-game METAR requests, or windshear system), lack of terrain functions (which can likely be overcome by using a custom terrain database - we haven't looked too much into this), and some issues regarding capturing throttle events - this is likely something Asobo has to fix, at least for us to implement a fully functional custom autothrottle and FADEC. Everything else is basically functional to write a complex aircraft with, albeit not as quickly without comprehensive documentation. (We're writing our own custom fly-by-wire and autopilot systems from scratch in Rust, since trying to wrangle with the default PID controllers is nothing short of a nightmare).
This leads me to two possible reasons why development is stalled on the PMDG side. Possibility 1 is that they are trying to use the low-level graphics API, which isn't fully fleshed out at first glance. We haven't looked into that at all, since all of our display code is written in Javascript using the Coherent engine (being an open source project, we don't have to worry about obscuring/compiling our code, so there's no downside for us here). Possibility 2 is that in its current state, the SDK, although mostly complete, is still a pain in the ass to work with, especially with little documentation. Rather than sink man hours into figuring it out now and doing it the hard way, they are likely devoting those resource to P3D instead, and will only continue development in MSFS once the SDK is less time consuming to work with. Makes sense from a business standpoint - PMDG is almost unrivaled in terms of quality, and they know people will gladly wait for their products.
I have heard, however, that pretty much every 3rd party developer apart from Aerosoft is getting almost no communication or coordination from Asobo. This can't be helping their development, either.