This question has been answered by absent42. See the first response.
&chunk=`MyTwitterXTpl`
The TwitterXTpl.html file should be in the core/components/twitterx/elements/chunks/ directory, but you should have chunk named TwitterXTpl in the chunks section of the Elements tree in the Manager (possibly in the TwitterX category). The chunk is what is actually used by TwitterX.

can anyone tell me where I might find some twitter tpl examples? Or for that matter, where I can find that default tpl?
It's in the Elements tree in Chunks>>TwitterX

My mistake, really. I shouldn't have even mentioned the file.