Condensation drains are indirect waste piping.

2014 CPC

803.0 Indirect Waste Piping.
803.1 General. Except as hereinafter provided, the size and
construction of indirect waste piping shall be in accordance
with other sections of this code applicable to drainage and
vent piping. No vent from indirect waste piping shall
combine with a sewer-connected vent, but shall extend
separately to the outside air. Indirect waste pipes exceeding
5 feet (1524 mm), but less than 15 feet (4572 mm) in length
shall be directly trapped, but such traps need not be vented.

Indirect waste pipes less than 15 feet (4572 mm) in
length shall be not less than the diameter of the drain outlet
or tailpiece of the fixture, appliance, or equipment served,
and in no case less than 1⁄2 of an inch (15 mm). Angles and
changes of direction in such indirect waste pipes shall be
provided with cleanouts so as to permit flushing and

Alrighty then.  Condensation piping that is less that 5'  or greater that 15' in length need not be trapped.  Condensation piping between 5' and 15' while requiring a trap, no vent for the trap is required.  Closet installations are almost always less that 5' and virtually all attic installs are over 15'.

Odds are that they didn't know any of that.  A trap is not a trap unless there is a minimum 2" column of water.

This was the first and the last time that I have seen this.

Not a good sign when the secondary isn't plumbed and look at that vent.  The storm collar is the sole support