That's a bit of a myth/misconception.

Last season, Siemian had an ALEX score of 1.0 (higher than Ryan, Carr, Wilson, Smith, Wentz, etc.)

ALEX or "Air Less EXpected" measures the average difference between how far a quarterback threw a pass (air yards) and how many yards he needed for a first down.

Siemian also had a positive AYTS (Air Yards to the Sticks) score - which measure the amount of air yards ahead or behind the 1st down marker on average completions on 3rd downs.

In addition, Siemian was ranked 8th for the 2016 season on converting on 3rd and long passes.

Trevor was less successful on shorter 3rd down pass plays - some of that having to do with the talent level at TE and slot, being under pressure a lot, the playcalling and it being tougher for a young QB to throw to the middle of the field (esp. against zone coverage).

What really killed the Broncos 3rd down conversion rate was the running game and games when Siemian didn't play.