No. In any whatsoever business, you never ever equate your market position based on daily, weekly, or monthly sales. You never claim to be #1 in the market if you are not actually #1 in the market. In fact, in some businesses, you can get in very big trouble for claiming you're the market leader because your shareholders can be misled by such claims. Sales are measured from product introduction to product discontinuation, so common sense should tell you that market share calculations must be based on cumulative/yearly sales.
I have no idea what market(s) Greenberg is referring to, so I'd love to know which territories the Xbox One is actually #1. Notice how he doesn't specify what #1 means; he could mean #1 in market share, #1 in accessory sales, #1 in amount of zombies killed on next-gen consoles etc. He could mean #1 in anything, and his lack of specificity is what ensures he never enters into legal hot waters with investors.
Greenberg's tweet brings up one of the big elephants in the room. Microsoft has a well documented history of being deliberately mendacious when they report Xbox sales performance, e.g. Xbox is the fastest selling console, Xbox*, US+UK= Market Leader, etc. Part of the reason why there's a general dislike of Microsoft 'personalities' is because they are almost never straight-forward when it comes to facts; there's always a twist and a turn and an asterisk involved. I can't remember which gaming host said it - it might have been on a podcast or a video - but when Microsoft were claiming they finally unraveled the mystery of the E.T. dump site, the person said something along the lines of "And this is why nobody likes you". They are habitual liars, and frankly, their inability to connect with core audiences is very much a result of them being dishonest in their business.
It's no surprise that the PS4 is #1 in Germany. The Xbox One sounded like an EU lawmaker's wet dream prior to the policy reversals. It was never going to do well with the data-mining camera and attendance check-ins, so even though the console has changed* in vision, the folks in Europe aren't interested in supporting the platform.
*= For now.
