If Continuous Press is enabled, Summon won't start at all from the key fob. That setting requires using the smartphone app. I'm not sure why they didn't implement the dead-man's feature on the key fob too, maybe it's not technically feasible, but in any case it doesn't work there.
Note that there's no need to double-click the fob to initiate Summon. Just hold the middle button for a couple of seconds, until the mirrors fold and the hazards come on. Then press the front or rear truck buttons to initiate.