Only the bits of Linux which are used are linked in, same as when you link together any program. The big advantage of using Linux is driver support - you can run a UKL application on baremetal, linking to the drivers needed to run on the target hardware.
My gut instinct is always that using traditional kernels in a unikernel way is a bit suboptimal because it doesn’t become a “library operating system” in the same way that Mirage does.
It's a significant discount from previous funding rounds according to Techcrunch: "Giphy last raised $72 million at a reported $600 million valuation at the end of 2016"