he left Microsoft before original Windows Phone 7 release.

Per his LinkedIn profile, he worked on it prior to release (as is stated on his profile). Even you have to be able to see what's wrong with that line of logic if you're as smart as you claim to be.

And he made his statement about mobile Microsoft system in 2015 (6 years later) exploting his status as of former Microsoft employee 6 years later.

He answered a question on Quora. He was in a position to do so because of his work experience on the WP7 team. He is, therefore, a reputable source of information regardless of how long he was at Microsoft, his role at Microsoft, the time between his departure and the Quora post, and post-Microsoft ventures.

If you discredit sources like this because they don't fit your definition of an information source then that's on you and you have, as you have so eagerly demonstrated time and time again, a huge ego issue. You feel the need to engage me to prove to yourself that you're right. You are the one acting like a troll with your immensely faulty logic and constant ad hominem.