Post the language you are concerned about. I don't use that service and haven't seen the terms, but it sounds like a pretty standard indemnity provision. If you post something that gets them sued, you have the defend them. This is really not unfair seeing as they didn't have any say in what you posted and shouldn't have to pay to defend a lawsuit brought about because you (allegedly) did something wrong. Now if it says you have to indemnify them for something unrelated to your actions that would be something different, but I would be surprised if that were the case.

In the real world unless they know the poster is wealthy or likely to have insurance coverage they probably wouldn't seek indemnity because collecting from a random shitposter is likely to be somewhere between tough and impossible, but there is really nothing wrong with them having the right to do so if they want.