Do you have any evidence for Skoda developing a platform on their own?
Audi and Porsche did do their own platform development, but that was never used by VW or Seat or Skoda and was for a completely different market segment.
https://www.autocar.co.uk/car-news/new-cars/skoda-handed-con...
Not developing as in starting from zero, but developing as in taking an existing platform and modifying/improving it for a new series of cars (in this case simplifying the platform to make it cheaper to produce, and targeting the very lowest end of the car market). Admittedly not a platform we're likely to see in 'the west', and sure it's nothing fancy, but I guess you have to start somewhere.
edit: reading back through the original thread, I agree that you are correct that Skoda is not developing a ground up platform that will be the basis for several VAG cars.
And it definitely isn't something which VW uses to base their own cars on.
So that a Golf platform is first developed by VW and then productized by VM and Skoda, but Skoda is already developing next year's platform which VW will use.