The WWE Superstar confessed his hypothetical demise during a lie detector test with his boxing superstar sib on Wednesday ... when Jake asked straight-up if Logan thinks he would come out on top if they faced off in the boxing ring.

Logan was surprisingly quick to respond ... uttering a simple "no" in response.

Sure enough, the whole room lit up in green ... signaling he was telling the whole truth.

The conversation comes at a pretty interesting time -- the bros teased a potential fight when it was announced the Paul vs. Mike Tyson bout was postponed due to the boxing legend's recent medical issues ... but realistically, the two have stated it's highly unlikely it would ever happen due to their mom's stance on the whole thing.

There were several other interesting moments in the clip -- the Maverick also admitted his former nemesis-turned-business partner KSI would also fall to Jake as well.

As for the younger Paul, Logan asked whether he felt he would actually beat Tyson in November ... and while Jake responded with confidence, the red lights flashed -- meaning he was fibbing.

Jake came clean on his stance ... saying he's truly nervous about the bout and isn't so sure how it'll fare for him.

Jake also addressed juicing rumors ... adamantly denying he is on performance-enhancing drugs -- which was the truth (outside of Viagra use).

Another L for the haters -- Jake said the Tyson fight is NOT rigged ... pointing to the fact it's a federal crime to do such a thing.

The lie detector test video ended up being an ad for Jake's new W men's care line ... which he spoke at length about with TMZ Sports earlier this week.