why would a radiowave that is reflected off the atmosphere (and therefore taking the longer route) be faster than a direct fibre cable?
In a vacuum, electro-magnetic waves travel at a speed of 3.336 microseconds (μs) per kilometer (km). Through the air, that speed is a tiny fraction slower, clocking in at 3.337 μs per km, while through a fiber-optic cable it takes 4.937 μs to travel one kilometer – this means that microwave transport is actually 48% faster than fiber-optic, all other things being equal.
At those time scales, the difference is so large, it was incredible what they were willing to pay to build these networks!
If you take one of the routes in the article, Chicago to Sao Paulo.
The distance is about 8,400km in a straight line.
According to https://en.wikipedia.org/wiki/Skywave a single shortwave hop can reach 3,500km, so 3 hops are required, or about 30ms.
The shortest commercially available submarine cable between the US and Sao Paulo alone is significantly higher than that (almost double), and it comes out of the east coast, so you'd still have to factor in the latency between Chicago and New York.
Even specialized low latency networks that mix wireless and fiber will still have much higher latency than the radio.
The tradeoff is that shortwave radio has very little bandwidth so you're restricted to simple signals.
Microsoft bought a hollow optical fiber company for a reason.
Generally they want to build the datacentres close to metro areas, by using hollow core fibre the radius of where to place the data centres has essentially increased by 3/2. This significantly reduces land acquisition costs, and supposedly MS has already made back the acquisition cost for Lumenisity, through those savings.
The immediate allure of hollow-core fibers is that light travels through the air inside them at 300,000 km-per-second, 50 percent faster than the 200,000 km-per-second in solid glass, cutting latency in communications. Last year, euNetworks installed the world’s first commercial hollow-core cable from Lumenisity, a commercial spinoff of Southampton, to carry traffic to the London Stock Exchange. This year, Comcast installed a 40-km hybrid cable, including both hollow-core and solid-core fiber, in Philadelphia, the first in North America. Hollow-core fiber also looks good for delivering high laser power over longer distances for precision machining and other applications.