WTC "start of phase" rulings?

Played a tournament this weekend with WTC FAQ rules and I was told that according to WTC you can't use the Dark Angels watchers in the dark ability for a 4+++ vs mortals against Doombolt. The argument was that Doombolt happens at the start of the shooting phase and that is the same timing where I need to declare if I want to use my watchers in the dark or not and since its my opponents turn he has priority and will use all his "at the start of X" abilities before me.

Watchers in the dark do not state you need to use them at any specific timing so I wonder if WTC has ruled that abilities like that need to be declared at the start of the phase, or is the start of the shooting phase not considered to be during the shooting phase and thus I cannot use the ability yet?

Im really confused because I can't find anything in any of the documents that say it should work like this but we checked with the TO and he agreed it works like this. They also mentioned it was a big discussion on the WTC tournament recently and it was indeed ruled to work like that.

Plot twist, I was actually the TS player and one of my teammates told me about this interaction and I immediately went to the TO and like I said he confirmed this. But it felt super scummy and I want concrete evidence to show any opponent before I would even consider claiming it to work like this.