Monk Unchained says the following about weapon proficiencies:
Monks are proficient with the club, crossbow (light or heavy), dagger, handaxe, javelin, kama, nunchaku, quarterstaff, sai, short sword, shortspear, shuriken, siangham, sling, spear, and any weapon with the monk special weapon quality.
There is an FAQ/Errata that is a bit dated in comparison linked on the Pathfinder SRD:
Q: If a weapon is specified as a monk weapon, does that mean that monks are automatically proficient with that weapon?
A: No. It means that they can use this weapon while using flurry of blows. It does not mean that it is added to the list of weapons that a monk is proficient with, unless the weapon description says otherwise.
Are unchained monks now proficient with monk weapons?
This came up because a player in our group wanted to use unchained monk BUT wanted to use a monk weapon 'seven-branched sword' which doesn't specify it.
Best Answer
Yes.
Regular monks weren't given proficiency with monk weapons. That FAQ applies to them. It isn't about the Unchained monks. As you note, it's old, and Unchained monks didn't even exist at that point.
Unchained monks are given a weapon proficiency with any monk weapon, hence the bit you bolded.