No, you're wrong. Show from the rules where 5 vs. 15 doesn't apply when one foul is a Dead Ball Foul.
The exceptions for offsetting fouls take Dead Ball and Live Ball vs. Dead Ball fouls into account. The exception for this play makes no distinction
because they are accounted for in later exceptions. The one that applies only mentions the yardage and nothing else. It is all-inclusive, Live or Dead Ball. Show me different. I'll wait.
Exceptions:
(1) If one or more fouls by one team includes a 15-yard penalty, and the penalty for the foul or fouls committed by the other
team is for a five-yard penalty without an automatic First Down, a loss of down, or a 10-second runoff (15 yards versus
five yards), or that is not a spot foul, the 15-yard penalty is enforced from the previous spot, and the five-yard penalty is
disregarded. Five vs. 15 enforcement cannot be declined by the team that committed the minor foul, except as described
in (2) below. See 4-8-2-h and 14-4-9-Item 3-Exc. 1 for dead ball fouls at the end of a half.
(2) If one of the fouls is a Dead Ball Foul for delay of game for spiking the ball and the opponent’s foul is a Live Ball Foul,
the team that committed the delay of game foul, in addition to Article 1 above, will have the option to decline the foul
committed by its opponent and be assessed the penalty for delay from the dead-ball spot.
(3) If both fouls are Dead Ball Fouls or are treated as such (14-4-9), the penalties are offset, and the ball is next put in play
at the succeeding spot, unless the Dead Ball fouls occur after the ball has been made ready for play, in which case 5 vs.
15 enforcement applies.