Please note that Susan's snark in panel four is hers and hers alone! I would have been happy leaving the point at panel three, but this is a comic strip, darn it. I had to end it on SOMETHING other than "can't we all just get along and drink lemonade together".
(That said, that is basically what one would have to do to get me to change my mind about this)
That said, there are dictionary listings for "opinion" with as many as six different uses considered to be valid and not antiquated by the dictionary they're in.
The opinion that opinions are never right or wrong is dependent on one of those definitions being the only one people use, and it's not. People express all sorts of things that they will claim are their opinions, and there are dictionaries that will back them up on it. It doesn't make what they said truly subjective and immune to scrutiny, but their use of “opinion” will at least technically be valid.
So deal with it, people who hate cereal stealing jet pack koalas! YOU'RE STILL WRONG!
EDIT: I wound up rewording panel two a bit. Susan's example was originally "I don't believe climate change is affected by people." Susan's point that whether climate change is affected by people is objective was the same, but it read like she was referring to whether that hypothetical person believed in something. The current version is more in keeping with people saying stuff "in my opinion, (insert something totally not subjective here)", which is sort of the point.