I think part of the confusion is that we're discussing two different rules at times.
The defenseless receiver rule is the one that has come into focus recently due to the NFL and officials emphasizing it over the past few seasons. By definition, Edelman was not a defenseless receiver, so the helmet to helmet contact, whether Edelman dipped his head or not, is all irrelevant when discussing this play.
The second rule is spearing, and that one is a bit trickier. Basically, is the defender using the helmet as a weapon? In spearing, it doesn't even require helmet to helmet contact. If the defender uses the helmet as a weapon to hit any part of the other player, it is spearing. So again, the helmet to helmet is irrelevant, whether Edelman dips his head or not doesn't matter. What matters is if Landry was using his helmet as a weapon in the hit.
Upon first viewing, I thought it wasn't a penalty because of the focus on the helmet to helmet contact on defenseless receiver, and in this case, Edelman is not a defenseless receiver. But upon further review, it's clear that Landry is leading with the helmet down. He does launch himself helmet first. Textbook tackling tells you to look at what you're hitting, and in this case, he not only hurt Edelman, but could have hurt himself too.
Edelman does dip his head as well but again, the helmet to helmet portion of the hit is irrelevant when defining spearing. Landry is looking down throughout the hit. That should have been a flag, and Landry is lucky he wasn't injured as well.