Here my 2 cents.
Maybe is not misinformation but a general lack of undertanding
Now that we have that out of the way, let's discuss some things misinformation can be:
Companies often use misleading marketing, exaggerations, or outright lies to promote products. That's misinformation. The effect often becomes part of the advice promoted in the hobby. Seachem Prime would be a simple example.
Some companies outright lie about a products ingredients. UWC Vibrant would be an example.
Industry personalities sometimes make claims that can't be verified or that contradict known science. Claims made by Claude Schumacher of Fauna Marin regarding the science behind "bolus dosing" would be an example.
Sometimes an error gets quietly fixed and the explanation doesn't add up scientifically or mathematically. Fauna Marin's Balling Light calculator is an example.
What is being steered away from in this thread may matter more than any of those.
Misinformation is also when someone with a misunderstanding keeps repeating it after being shown they're wrong. This type of misinformation also often works its way into the fabric of the hobby.
Honest mistakes happen, and that's unavoidable. Many of us have at one point or another been the source of, or passed on, misinformation. It's not the same as someone who keeps doing it after being told they're wrong, refuses to let corrections stand, and builds authority through repetition instead of evidence. Calling something an opinion or a hypothesis and wrapping it in hedges doesn't exempt it either, once it's been shown wrong and gets repeated anyway, the label stops mattering.
Most people who are informed that they are spreading misinformation take steps to correct both their understanding and the record moving forward. That correction is easy to spot when it happens. It's just as easy to spot when it doesn't.
Redefining it doesn't change what it is.