Author here.
Firstly, this is a rather silly argument in general. One could say that Embeddable Common Lisp and Guile address the same niche and all developers from one project should consolidate in the other.
I have not used Guile to any extent, nor was Janet really influenced by it, but the comparison has been brought to my attention several times in the past. Janet is not scheme nor common lisp, so there are of course many differences that I cannot address here. However, as an extension languages, there are several things that I believe Janet does better than Guile.
Guile is not an ideal extension language for C projects (this is distinct from an extension language for GNU projects). Firstly, they address garbage collection with the Boehm GC, which takes some control over your runtime and is not known for great performance (the runtime/non standard C is the bigger issue). This was of course a decision made by Guile devs to make it easier to write correct bindings to C libraries in Guile, but it is not without it's downsides. Secondly, the project is not easy to embed in foreign build systems. Janet is more like Lua - built in tracing GC that is not conservative nor platform specific, no external dependencies beyond libc, and can be added to a project as an amalgamation (a single C file and 2 header files). This has enabled people to get Janet running more easily on a variety of platforms like mobile phones and even recently someone has been doing some work on getting Janet running on the Nintendo Switch. I believe Janet is also significantly smaller than even the smallest build of guile. Basically, Guile is not simple, which is (IMO) antithetical to an ideal extension language.
At the same time, Janet should feel like a much less minimal language compared to Lua. While core Lua limits itself to pretty much only standard C, Janet goes a little further to provide some abstractions over different platforms (that can be turned off via compile time options).