You illustrated the point better than I had the energy to do-- I simply don't care enough to be that eloquent because the majority of what I get is nonsense in return. So instead of wasting my time, I chose to take the lazy route and encourage people to come to a consensus on this one key point so that the point you made would become irrefutable.
Neither trans nor cis are slurs by default, but anything can be a slur if used in a derogatory manner-- I think some comedian had a bit about this, but I can't recall who and I'm sure even if I did there's a good chance someone would give me a nasty label for saying such comedian made a good point....
But the same people that say trans can be a slur, cannot deny in good faith that cis cannot be a slur. They are both simple classifiers with no basis on prejudice. But some people will still argue they are slurs simply because they are used prominently in derogatory contexts.