Actually, the repo was already updated to work with vtk 9.1 and wslink >= 1.
Your code base was probably from an older version of the repo as twisted is not referenced anywhere anymore.
BUT, that piece of code is really meant for the demo not for real deployment which can mess up the connection back to the server.
Anyway, either version should be fine as long as you have consistency on your full stack. If you want to stick with twisted, just pre-install wslink<1.0.0 before installing VTK < 9.1.
If you want to use VTK >= 9.1 you must use wslink >= 1.0.0.
HTH,
Seb