I do not know if that is true. I always used to think differently, but I have never tried it myself. This is how I think it should be able to be used as modem.
Choose in the menu Internet / Zugangsdaten (in english should be something like "access data", or "credentials"). In the first tab "Internetzugang" ("Internet access" or similar) there is a dropdown labeled "Internetanbieter" ("Internet provider"). Choose "Vorhandener Zugang via LAN" ("Existing connection via LAN"). Then, a little further down, click "Verbindungseinstellungen ändern" ("Change connection settings"). In the area showing up you activate PPPoE-Passthrough: "Angeschlossene Netzwerkgeräte dürfen zusätzlich ihre eigene Internetverbindung aufbauen" ("Connected devices are allowed to build their own internet connection"). After accepting the settings the Fritzbox should behave like a modem. Don't forget to deactivate other Fritzbox services like DHCP if you want only a modem.
Caveats:
1.) I did not try this out myself. I intent to at some point in the future though, as I'm on a similar quest: An APU.2 will be my new router, and I will test it with the existing DSL line and the Fritzbox as a modem, before I switch over to cable (cheaper, more bandwidth).
2.) The description is for Fritz!OS 7.01. Other versions may look a little different.
3.) I did not bother to change my Fritzbox to english to read the labels. So beware of some inaccurate translations of the labels that I found in my box' web interface.