tablex is indeed a regular function, much like your usual calc.rem(5, 3) function; tablex is, however, special in that it returns content you can display in the document. But that's about it. Ideally, tablex would be turned into an element, but currently that's impossible in Typst (no syntax for that). So you can't use show rules on tablex at the moment.

One way around it is to simply specify default parameters for tablex. For example, you can write:

// simulate a set rule
#let tablex = tablex.with(stroke: red)

Or

// simulate a show rule
#let old-tablex = tablex
#let tablex(..args) = {
  let it = old-tablex(..args)
  block(stroke: (left: red), it)
}