After basically wasting the afternoons of a week, I didn't manage to get the tool work. It may matter that I've tried to build a safe Linux on an USB stick for the job and that ate up a lot of time too (and the result was far from perfect), but this solution to obtain the recovery phrase is not for me.
Now I know that I better don't mix Monero with Ledger. Thank you very much for opening my eyes, I may have made a mistake otherwise.
Not for or against using the Ledger...
But in the past I found this tool to produce the same 25 word see from the BIP39 24 word set:
https://github.com/Coinomi/bip39-coinomi/releasesHave not tested it in ages against a Ledger, but it certainly worked in the past.