The point isn’t how, but that the idea isn’t in the words. If I held that view, I’d be bringing in assumptions into the words that aren’t actually there in the words. This is true even if the author actually had those thoughts. The words simply don’t carry that information.
The modesty here is about saying "We're not sure if this is generally surprising for everyone, but we were surprised". "Surprisingly" is a bit less conservative to me, its more of a "We think this is generally surprising for the field". I would personally use it only if I knew something has already surprised multiple research groups