I'm pretty sure Dr. David Brinner is a reference to Dr. Brinner Ghost Psychiatrist a fake MS Paint Adventures forum joke and that letter is just another reference to Hussie's other non-Homestuck works. But people keep reblogging that post believing Dr. Brinner is Dad Egbert which makes zero sense if they stopped to think about it

I did not get the reference, thank you and everyone else that told me for sharing.

~Mod T