There are many RT-PCR LDTs. It does not take very much to create a real-time RT-PCR LDT for COVID-19; two weeks ago the CDC posted a comprehensive description of the process on their website, but it was not a secret before then. All you need is a clinical-grade RT-PCR machine, synthesized primer/probe DNA (which many labs are equipped to do), and quality reagents.

The difference here is that the Roche instrument is pre-approved for clinical use both (1) in the US and (2) outside "high complexity" CLIA labs; the instrument is already widely deployed to hospital labs; and the instrument is highly automated compared to the basic 96-well RT-PCR machine, with a robotic stage that allows it to run thousands of samples per day with less manual work.

The WHO does not have a massive stockpile of complete test kits, and to my knowledge the countries that have scaled up testing are not relying on the WHO for anything but information. Any kits the WHO provides would need to run on an existing RT-PCR machine anyway.