This is because Frigate's are Conditional Battleline in Barak Zilfin, meaning they lose the Behemoth Keyword and gain the Battleline Keyword.
For the purposes of the Battle Regiment and indeed any other Battalion, they are thus considered to be Troops.
Edit due to thread being revived: core rules FAQ voids my response below, as the clause surrounding benefitting once per phase has been removed.
Original: I think it comes down to what the word "benefit" means in the context of Age of Sigmar.
As has been pointed out to me previously on this, apparently "benefit" was the old way of saying "receive" a command ability, though now for AoS 3 we have receive and issue making things a bit clearer.
In this scenario, what does "benefit from" mean?
Does it mean receive, and thus it can be stacked, or does it actually mean benefit as per the definition of the word, and thus you can only ever have +1 attack from it, regardless of how many times you actually stack the ability?
I play Soulblight, and I'm the type of player to try and play every rule to my advantage, however I find this one to be a bit of a stretch purely RAW, because to me "benefit" means benefit, and you can't benefit more than once, meaning you can't ever have more than +1 attack from this ability. Even my WAAC brain would feel slightly dishonest using this one, unless cleared with a TO ahead of time.
Unless you can provide the core rule for abilities not being able to stack, then this is not correct.
The reason the fumigator doesn't stack is because of the way it's worded, it's simply a check if enemy models are within range and then the effect is applied.
You can stack Mystic Shield. You can cast debuff spells such as Overwhelming Dread or Fading Vigour and if they proc twice from Locus of Shyish, you are free to resolve the effect for a second time on the same unit, providing -2 to Hit / Attacks respectively.
