5e said no such thing. That's you assuming.
"After assigning your ability scores, determine your ability modifiers using the Ability Scores and Modifiers table." (Basic Rules, page 7)
"Because ability modifiers affect almost every attack roll, ability check, and saving throw, ability modifiers come up in play more often than their associated scores." (Basic Rules, page 57)
"The three main rolls of the game—the ability check, the saving throw, and the attack roll—rely on the six ability scores. The book’s introduction describes the basic rule behind these rolls: roll a d20, add an ability modifier derived from one of the six ability scores, and compare the total to a target number." (Basic Rules, page 57)
"For every ability check, the DM decides which of the six abilities is relevant to the task at hand and the difficulty of the task, represented by a Difficulty Class... To make an ability check, roll a d20 and add the relevant ability modifier." (Basic Rules, page 58)
"Intelligence measures mental acuity, accuracy of recall, and the ability to reason." (Basic Rules, page 61)
So when I say "D&D 5e said it's a -3 modifier to Intelligence-related checks to test the character's mental acuity, ability to recall, and reason..." I'm right. And when you sit there and claim that the rules say that:
It does set low int = low ability to reason.
And in the same breath say that my absolutely correct paraphrase of the above-quoted rules is just me "assuming," you are not arguing in good faith. It doesn't take an IQ-test taking frog to see it either.
Why would it be impossible according to the rules you put forth? I wouldn't allow it. That's your construct that creates the situation where the frog should be able to.
You are confused. In this game, the DM determines if an ability check is appropriate (DMG, page 237). That I do not force players to play - or only play with players who play - Int 5 a specific way ("specific" as compared to playing it however they like) does not mean frogs get to make Intelligence checks to take IQ tests. It's silly to even have to explain this to you. I don't know where you're even coming up with this stuff anymore. But I do know why.