Google, Amazon, Microsoft, and Apple all have B2B SBUs. That doesn't mean they're necessarily B2B companies.
I think in the end it makes sense to classify Microsoft as B2B as:
a. For much of its history it has been predominantly a B2B company for its products that got traction (O/S, Windows, Office are used by consumers, but the big $ customers of these products have traditionally been IT companies). With the exception of XBOX, most of its consumer products have been big money losers.
b. Even today, the majority of Microsoft profits are from enterprise or SMB sales of its O/S, Windows, and Office products. Its core consumer products (MSN etc.) are largely money losers. So if you think of MSFT as a business, it mainly makes money off of other businesses.