PPPoE (or really PPPoA) uses 1483 (From the CPE) to encapsulate the PPPoE session within ATM. Since this (PPPoE) is layer 2 broadcasts will be past from CPE to PE and could cause problems. Normally Layer 3 devices can offter broadcast delmiters where BCAST traffic is compartmentalized. I think this comment was made due to the fact that L2 Bcasts would be past to the PE (Provider Equip).
Hope this helps you,