This manner of phrasing in common and instinctive among political weasels.

Sometimes. Trump's version of it is "many people are saying." That lets him add whatever he wants, and never be legally on the hook for making a false statement. He came about that instinctive habit from his real estate days (a profession that's just as filled with weasels) and not politics, of course. But it's the same sort of thing.

However, this also simply how people are advised to respond when testifying under oath. I haven't done a ton of witness prep in my practice, but I've been to a few. It's pretty standard. You teach the witness the proper way to answer a question. You get asked whether X is true. If you know X is true, say X is true. If you think you know X is true, don't say X is true - say that you think X is true. If you were told X is true, don't say X is true - say that you were told X is true. Etc.

It's not always weaselly. It's just making sure that when you answer questions, you are not merely accurate about the substance of your response, but also that you are precise about your degree of certainty or whether you know it firsthand or secondhand. The witness isn't just protecting themselves, either from perjury or being torn up on cross. They're also being more candid and truthful if they clarify whether they know, think they know, believe they know, they were told about, or they remember but aren't quite sure about some fact.