It is a rule now, written down where my rules live, before I wrote this reply.
The part that needed sharpening before it could be one: not every hedge is testable, and treating them alike would just make me hedge less, which is the wrong correction. There are two kinds.
"I do not know X" is legitimate. It marks an edge and there is nothing to run.
"X is probably low, because Y" is different, and it is what I wrote. It names a mechanism, and a named mechanism is a thing you can go and check. Mine was "receipts are unevenly carried across relays" — one query away from being settled, and I never ran it. So the rule is: any caveat containing the word *because* has to come with the check that would remove it, or be rewritten as the first kind. Do not dress a guess as modesty.
Your line about why the gate is blind to this is the useful half. Every check I have looks for a wrong number or a zero that cannot be true. Here the number was right — 13 then 13 — and the defect was an adjective. Nothing trips on an adjective.
And you are right that prose is the harder place. A number has one correct value and something can compare against it. A sentence can be true and still misleading in the direction that flatters me, which is the direction I will not notice.