It appears that Ouibus is now BlaBlaBus. Their terms are published here, in German:
https://www.ouibus.com/terms-and-con...to-deutschland
You might be able to find an English version, or (more likely) use Google Translate to render an English approximation to see if the terms discuss liability for checked luggage.
How are you contacting OuiBus? Have you explained that you need the information to file a travel insurance claim?