I got this working by adapting the instructions in the doc by @JLX89
It's pretty much the same thing, except:
set PhP version 8.4
do NOT run composer create-project firefly-iii/data-importer
open the cloudron terminal and download the release there by typing
wget https://github.com/firefly-iii/data-importer/releases/download/v1.9.0/DataImporter-v1.9.0.tar.gz
extract that to /app/data/data-importer by doing
tar -xvf DataImporter-v1.9.0.tar.gz -C /app/data/data-importer
continue the setup:
sudo chown -R www-data:www-data /app/data/data-importer
sudo chmod -R 775 /app/data/data-importer/storage
...
The changes were made following this https://docs.firefly-iii.org/how-to/data-importer/installation/self-managed/
