You are. It's a slur. It is not an accepted term in the medical community.
Edit here's an actual definition from https://www.merriam-webster.com:
dated, now offensive : affected by intellectual disability : INTELLECTUALLY DISABLED
NOTE: The term is increasingly considered offensive. The use of intellectually disabled is now preferred over in medical, educational, and regulatory contexts, as well as in general use.
Further, you use it as an insult specifically in that you suggest someone of a certain political preference is intellectually disabled.
Essentially you are combining someone's medical situation they did not pick or cause, with someone else's political interests. That's fucked up.
Edit this is like if you used the n word then finished a comment saying "I'm not insulting anyone. I'm using it my own way". You don't get to do that, that's not how slurs work.