I think you're misunderstanding what is going on there.
Alex Jones didn't say 'In my opinion sandy hook blah blah blah'. He said 'Sandy hook blah blah blah'. He presented those things he was saying as facts, not as something that was 'just his opinion'. Under oath it's a similar rule, but does allow more leeway regarding the persons knowledge. [https://leginfo.legislature.ca.gov/faces/codes_displaySectio...]
And what the Judge is telling him, if you do that, it doesn't matter if you think they're true or not, what matters is if they are actually true. Or at least under oath, it's plausible they he believes they are actually true and it's not all delusions or whatever.
Especially if you're saying them in a large venue, with real impact on real people.
Actual truth is an absolute defense for libel in the US. If what you're saying is true, you are not committing libel. Period. even if you think it's actually false when you're saying it
Thinking something is true if it is actually false is not enough in 99% of cases, assuming it is being presented as a fact and not an opinion (which requires no verification and is generally protected from libel in the US).
There is a bit of an escape clause, which is libel generally also requires that the false statements were made with some degree of recklessness, negligence, or maliciousness.
As in, they were not only false, but the person saying them also didn't do the basic amount of checking appropriate in the circumstance to avoid harm before saying them.
Like if someone goes on TV and says 'OJ died today, he was murdered by John Smith' and they never bothered to verify it and just overheard a hallway conversation or whatever. John Smith, and probably OJ would have pretty solid libel cases.
If someone did check and got it confirmed, say with OJ's agent, or the appropriate law enforcement authority, they'd generally get a pass, as they weren't reckless or malicious. It was an honest mistake based on information that should have been sufficient to determine if the fact was true or not. The law enforcement authority or agent might be in trouble though.