When it says there's not such thing as an off-hand attack for monks striking unarmed, it means that you adjudicate their off-hand attacks as if they were main-hand attacks; they get full Str to damage and don't suffer the half Power Attack damage. But they are still, technically, your off-hand attack for TWF and would apply as such for anything demanding an off-hand attack like feats or abilities. For instance, Two-Weapon Rend requires you to land a hit with both main-hand and off-hand weapon. You can still do this even if you're a Monk fighting Unarmed.
Next thing to consider is that the TWF feat doesn't grant you an additional attack; it simply adjusts the penalties from -6/-10 to -4/-4 for non-light off-hand or from -4/-8 to -2/-2 for light off-hand. FoB, at lvl 1, does the same thing; it doesn't "grant" you the extra attack, it simply lets you "set" your penalties to -2/-2 (regardless of what kind of weapon(s) you use). So, since you're not using the FoB class feature and (presumably) you don't have the TWF feat the standard way, your Unarmed Strikes would be adjudicated as if both were main-hand weapons so -4/-4 for both and full Str and Power Attack bonus on both as well. By contrast, if you had a pair of Kama (which don't benefit from the "off-hand as main-hand" benefit), you'd be attacking at -4/-8. You could also use a weapon in your main-hand (ie. Temple Sword) and make off-hand attacks with Unarmed Strikes and still be at -4/-4.
