They absolutely recognize that ESPs are a commodity, even reselling delivery to ExactTarget, Responsys, etc, as the "pipe". It's hard to believe your reasoning when the industry leader is saying something different.
LinkedIn, Facebook, Twitter, etc do not use Sparkpost, they use their on-premise installable software that many ESPs use and pay tens or hundreds of thousands for, which indeed is a dumb pipe MTA. Those big companies still have a full-time staff managing delivery and infrastructure. It's basically a replacement to Postfix, not a full fledge, multi-datacenter, multi-tenant hosted application to support many thousands of concurrent customers.
A hosted "product" is a different animal. Sure, it needs to have great delivery, but the work that goes into making it easy to troubleshoot when you have issues, minimize developer work, and bring useful data back into your application is something else. This only becomes painfully obvious when you deeply rely on email for your business.
There is a reason why Sparkpost left Postmark out of their comparison. We have the same data sources (eData), and we came out on top.
[1] http://pages.sparkpost.com/Big-Rewards-WP-Download-Landing-P...