The Conclusions do not Derive from the Facts
"it’s very, very clear that Amazon and Microsoft own the cloud."
Er, no that's not what your facts show. You said:
"Nearly two years ago, Gartner analyst Lydia Leong found AWS to have five times the utilised compute capacity of the next 14 competitors combined. (...) More recently Leong told me that AWS’ market share lead has grown, not shrunk, in the past two years."
When the market leader has 5 times the market share of the next 14 combined, and that lead is growing, then Amazon "owns the cloud" all on its own.
Microsoft is only 3 percentage points ahead of IBM. Compared to Amazon's lead, that's virtually nothing. In other words, IBM (7%) and Microsoft (10%) are peers in the cloud. Neither is really that far ahead of Google (5%), Rackspace (3%), or Salesforce (4%). Meanwhile, Amazon is at 28%.
Or let's put it this way, the gap between Amazon and Microsoft is roughly 2-1/2 times that of the gap between Microsoft and Rackspace.
If IBM is telling a few nose stretchers about market share, then so is Microsoft. IBM has far more right to lump themselves into "the big 3" than Microsoft does to claim to be peers with Amazon.
"So it’s a two-horse race, however you quantify it"
Not according to the numbers the article uses, however you quantify it. Here's what the facts in the article supports - Amazon owns the cloud, and everyone else, including Microsoft is an also-ran in a herd trailing far behind the leader. It's a one horse race.