It's an open ISA for vendors to use. If they want to keep everything else restricted and closed source, they can do it. RISC-V only means the ISA is open source.
SiFive incorporates closed source hardware and firmware from various partners, and perhaps some of their own too.
A lot of vendors are going to see it primarily as a way to save money by avoiding an ARM license. Many RISC-V products won't be any more open than the alternatives using an ARM SoC. Can make entirely closed source RISC-V hardware. Can make proprietary ISA extensions to it too.
you're of course entirely correct, but SiFive also wants to capitalize on intangible "freedom" (even their SoC names have that!). they can talk the talk, but then they have to walk the walk, too
that "freedom" includes the freedom for other vendors to make proprietary designs (and that's fine) as well as the freedom for developers to use RISC-V designs from the people who started it all. the latter does in fact include the freedom to share the SoC manual