The current mainline Linux kernel being worked on is version 5.7.
The reason why the original text is confusing is because most readers will assume that it is talking about the mainline Linux kernels, which is obviously not what David Hamner is referring to, which is why the text should be changed.
I assume that David Hamner is talking about Purism’s custom kernels in the Librem 5 releases, not the mainstream Linux kernels.
I assume that Purism takes a recent mainline Linux kernel and adds its custom changes to those kernels in its Librem 5 releases. Then, it works on getting those changes upstreamed to the mainline kernel. (Someone who has more knowledge please correct me, if that isn’t how it works.)