As a paramedic I can tell you that you don't need to hit your head to suffer a concussion.
"Coup - contre coup" is enough to get it done.
Basically any force that causes the brain to slam into the inside part of your skull causes a concussion.
Mahomes suffered a concussion to me no question.
And I'm not surprised he has a neck injury as well given tackle we see on tape.
He has 3 injuries that we know of:
1. A concussion
2. A neck injury
3. A toe injury