While I fully agree that the name is terrible (maybe we should start open sourcing product names?), the name is “Xournal++”, not “Xournalpp++”. The title here is wrong.
Looks like the name is "Xournal++", but the github repo name is "xournalpp".
They have the pronunciation written as one of the first things in the linked article: /ˌzɚnl̟ˌplʌsˈplʌs/.
Nobody is going to carry a pronunciation guide on the off chance they might want to recommend a tool to a friend.