Mounted Combatant: How does reach of attack interact with the feat’s deflection?

I was playing a campaign, and my DM attacked the mount. Imagine this is the battlefield (seen from above); I was on the C, my mount is large (A), and the monster is M:

# # # # # # # # # # A C # # # # # A A # # # # M # # # # # # # # # # # 

When I said “I deflect the attack to me,” the DM said “But the foe does not reach you, which means you can’t pull the attack to yourself.” That got me thinking. I’m wanting to know how this works in this situation.