I can't speak for the author, but many systems programmers look at declarative/markup-based UI as a kind of black magic that you can't trust and that will get in your way sooner or later by not leaving a sane way for you to address some project requirement.

We/they are often way more comfortable working with the kind of programmatic widget and graphics library that we/they might write, and whose behavior we can debug and trace with our usual tools, then somebody else's weird new declarative/markup syntax that obscures what's actually going on behind the scenes and frustrates our tooling and workflow.

And this instinct traces back many decades, for as long as visual RAD tools and declarative UI syntaxes first started being introduced by big vendors.