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But thanks anyways for a late completion of your own question.
Or to clarify your rushed interpretation...

given that iOS pages, numbers, and keynote have all been able to use different stock fonts since they have been available on iOS using the paint/format/style button and these updates specifically call out the ability to use the iOS 13 feature of using third party custom fonts, but I digress.
 
Ok, I can’t think of one shortcut to make use of this but my brain is like “this is a big feature but you lack imagination”.

Well, I've many! I use a lot of shortcuts to track/log different things like whenever I upvote a reddit post, it goes into a google spreadsheet via IFTTT, I log my meetings with people with notes which goes into my journal, I log medicines using an app called Round, I log my habits (getting early, going to the gym 4 times a week, calling close friends at least every 6 weeks), etc. And of course, the most common one to log expenses.

A lot of those use apps or go though IFTTT to google sheets (which I don't like). They all eventually end up in my Numbers in iCloud as I prefer the interface and it has almost all features I need. Exporting from Sheets or saving to CSVs from shortcuts were workarounds until now which was kludgy at best and one had to remember to do it. Now with shortcuts support, we can directly add data to the spreadsheets ourselves which will avoid the middlemen. Plus, if there's no network, this will work offline as it's in iCloud.

It's still not perfect though. There are two big caveats with this so far:
  1. It opens the app Numbers when adding a row as opposed to just adding in the background which could make some workflows difficult.
  2. To use the action, one has to use the action "Get Files" which means it'll either show a document picker always OR the file has to be stored in "iCloud->Shortcuts", it cannot access files in "iCloud->Numbers" folder. One workaround to having the file be automatically chosen is to use Dropbox instead but if one is moving a file to Dropbox, might as well move them to another folder in iCloud itself, right?
  3. Another minor caveat is that it cannot yet read back the data from a Numbers document except through a little hack. Although it doesn't matter to me as I'd rather do analysis in Numbers itself but some people miss that.
 
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