Well, there will probably be ways to tell apart BU nodes from Core nodes for a while.
At least, a BU node that wants to pass off as a Core node must get some switches to identify as a Core node, and it must also offer the same interfaces (CompactBlocks) and refuse the BU-only ones in the same way that a Core node would.
That's a lot of effort for questionable benefit. Any attacker already has known BU nodes mapped out by their IP addresses, and can DDoS your node no matter what face it presents.
A much better way to minimize any DDoS impact is to get more BU nodes up and running.