You think this way because you keep having the regular surprise definition in mind. My variant rule doesn't invoke ''threat'' at all in its text but instead says : ''Any character or monster that doesn’t notice a deception is surprised at the start of the encounter.''
I assume you mean to say, "at the start of combat." Does this rule apply to
any sort of deception? Wouldn't it have to be an effort to convince the other party that you are not a threat? Do you not let players decide for their characters whether they are convinced by such efforts or not? As a DM, I would never tell a player that their character is convinced of something by a Charisma check made by a NPC.
If the party talk to NPCs who consider them as friendly, they don't perceive them as a threat anyway, that is the very reason why it rely on deception instead. Think how sucker punch often happens, when one is caught offguard because it never expected the other person to attack it while talking.
To me, a sucker punch is a matter of winning initiative. Both parties are aware of each other, and one decides to attack the other. Initiative is rolled. If the aggressor wins initiative then she gets in the first punch because her target is slow to react. If not, then her target might give her a quick jab while she's still in mid-swing.
The same way you do it for regular surprise rule to determine if you notice a threat, by having an contest. Here its a Charisma (Deception) vs Wisdom (Insight) rather than Dexterity (Stealth) vs Wisdom (Perception) normally used.
This assumes that the true intentions of creatures are automatically known unless they are making an effort to deceive. I would rather that their intentions remain unknown unless you make an effort to determine them. I say this because usually when you notice a creature then you aren't surprised when it attacks. If this depends on being able to determine that they intend to harm you then it must be fairly obvious when they do, only necessitating an Insight check if, like your PCs, the creature is attempting to hide its true intentions. My problem with this is that it creates the, to me, odd situation that, as a player, I will always be surprised unless I can first positively determine that my attacker means to do me harm, i.e., that in the absence of that information, because I failed an Insight check, I cannot decide that my character nevertheless remains suspicious of the creature's intentions and the possibility that it may indeed be a threat.