At least I wouldn’t have a hissy fit about a family member using my name - that truly is unpleasant.
It’s hard for us to know exactly why that choice was made, and what message it was intended to send. It might of course have been a simple message of how much Elizabeth meant to Harry.
Given it came from a couple who had felt the need to publicly clap back at statements made by the Queen herself that she didn’t own the word “Royal” and that “service is universal” it’s also quite possible that it was a vindictive choice to show her she didn’t own her family nickname either.
If it’s true that the person involved and who knew what was really going on was unhappy rather than flattered, it suggests the latter is closest to the truth.