Not true. Microsoft PR is the only source of Xbox sales information whereas Sony and Nintendo's financial statements cover this is detail. Sales numbers. Revenue. Profit Margins.
Sony has done shady things like combine PSP and Vita shipments when Vita wasn't selling, and they combined PS3 and PS2 numbers just when it was convenient to obfuscate lackluster PS3 sales as well. Even now they give us a PS4 number only, not PS3.
Nintendo, as far as I know, does release a lot of numbers. Dont follow them closely enough to say if they've cut back or not.
Anyways, if it's not illegal MS can release however much sales info they want. They also for years were the most forthcoming with NPD numbers, tweeting X360 sales month after month (usually GAF worked out the other numbers based on what Microsoft gave out publically), yet I dont see you giving them credit for that. But, then again we're back to when 360 was doing well it was easy to give out numbers. Even today I think they are pretty key, it's clear NPD has banned anybody from giving out numbers, but MS still gives a percentage increase in sales that GAF calculates the number from. Nintendo sort of does this as well IIRC.
I guess MS does hide things better within their divisions, I dont know exactly how much Sony breaks out gaming anymore, I know they used to have a gaming division. Nintendo obviously cant help but be gaming only so by default we get more insight there. I dont entirely blame MS for being cagey, the gaming media seems to relish reporting bad Xbox news/sales..
I dont see why it really matters. Other than to nerds like me...