All Ttk widgets can be created using tcltk, except ttk::scale. I do not know whether this is an oversight or wether this is inteded.
In the former case, I can provide a patch, creating a simple function:
ttkscale <- function (parent, ...) tkwidget(parent, "ttk::scale", ...)
It was intentional. It was not needed within R, and nor has anyone requested it in several years.
In any case, you demonstrated the falsehood of
'All Ttk widgets can be created using tcltk, except ttk::scale.'
by the code you show.
Would you elaborate on why it is not needed? Is it because people can call tkwidget() directly? Indeed, the lack of ttkscale() is not a big deal, but AFAICT all Tk and Ttk widgets have their dedicated helper functions. Consistency is a good thing for users.
I think that if nobody requested it, it is because people use tkscale() without wondering, while ttkscale() would provide an alternative more integrated with native widgets.