Yes, they both take the elements used to create the website and builds a final site html image that can be uploaded to a site and served.
Jekyll/Ruby now is currently the most popular because it’s easier to develop site (with plugins and gems) and github will host, for free, your Jekyll sites, if you develop it on their platform.
A Jekyll/Crystal doesn’t have to be absolutely faster than Hugo/Go, it just needs to (and will) be much faster than using Ruby. The more compatible it is to Jekyll/Ruby the more attractive and easier to use it will be to Jekyll/Ruby users.
Just as Rails created mindshare for Ruby (which some people still don’t know the difference between) a Jekyll-like/Crystal combination can do the same for Crystal.
And since web development is a significant commercial enterprise, this could attract more developers and $$ to Crystal.
Crystal needs at least one something it is widely seen to be useful for, and this seems a natural fit.
Here’s a list of most (all) known static site generators, and platform information.