> At least within my organisation, we do not design anything up front. We’re agile

Agile doesn't mean forgo design completely.

> We don’t think about proper api modelling. We’re agile.

See previous point.

> We also do barely any testing. We’re agile.

It's difficult, if not impossible, to be agile without testing. If you want to move fast, you want to be confident that your latest change didn't break anything.

> We do rewrite the UI a dozen times based on user feedback. After all, we’re agile.

Sounds like you built a complete UI before any users could give you feedback. That's the opposite of agile.