You should read the actual questions on an I-485, they don't rely on declarations that any given organization is "terrorist" or anything like that.
Now, I think you're right that the statements are only required to be true at the time they were made. That said, if I can get you to look at the I-485 which one fills out to get a green card, and focus on the questions in Item Numbers 43.b. - 43.e, which cover what is essentially material support for a terrorist org, you will see that all of them relate to the group's actions. You can read the questions yourself here on p. 16 of the PDF for an I-485:
https://www.uscis.gov/sites/default/files/document/forms/i-4...
So the only way the truth of someone's statements could change over time would be if the groups they had given material support to (e.g. money or recruitment) had only engaged in things like assassination, kidnapping, hijacking, sabotage, destroying people or property with weapons or dangerous devices, etc. after the person had already turned in their I-485 form.
This isn't very likely to be the case for various Palestine-related organizations that are doing armed resistance, since that armed resistance has been going on for a very long time now. But I suppose it's hypothetically possible in the abstract, and I presume one would argue this at an immigration hearing, since they have to establish immigration "fraud" and parts of that depend on what you knew and when you knew it.