Why was this counted blunder?

Oh then I guess I will upgrade it next week. 

You don't need it. If you've got Platinum and access to Stockfish Analysis 20 moves deep, surely that is at least a dozen moves deeper than Mr Coach.

Wait, how do I access stockfish analysis?

You bring up the game you've played, and after a few seconds, it says Analysis, on the right, next to Game Review.

Oh that is stockfish. I see.

Stockfish Analysis gives you the best 3 moves at any time in the game, about 20 moves deep. You can even put in your own moves that you didn't play in the game and it analyses them too.

So you can put in any move in any game, whether you played it or not, and it analyses them to the max.

Okay thanks!

If you want to analyse a move you didn't play in the game, make the move on the board, at that position, and it will analyse it.

Actually I just realized you are 2000… I thought you are a beginner .. I guess I will never trust my senses again.

That’s the most helpful thing 

Yeah - I should change my username.

That's a touch of realism. The game is already lost for black and so no move by black can alter the result. Therefore black can't blunder.

True.  At least 4...Qb6 was not marked as Excellent, as I have seen major material hung for zero compensation in losing positions marked as Excellent on previous occasions.

Ho yes that's a very good tool. 

I am using it often with puzzle when I don't understand why a solution is not accepted. You go in analysis and play with your move then look at stockfish reply, play one of the best move and see how it goes. 

On this game it is what I did, to check if the rook sacrifice could work. It does not stockfish just move the knight in F8 to counter the mat and whatever I tried to do on the next few moves the evaluation stay at -2.5

It's a bit random. I've been trying to tell people it's all more or less random but some don't like being told that. I've seen excellent moves that drop the win, where other moves don't. I got two great moves and a brilliant in one 5 minute game the other day and they were just nothing special ... like moves a half decent player would make on a mediocre day.

I've just realised, it's because I dropped 150 grading points again and I'm playing at around 1500 in blitz. It gives you more of them when you're lower rated.

Yes, also see this:

Excellent and best should have been close, but ..

Right.  Not only should Excellent and Best be close but Excellent should also be excellent!