Recently I constructed a Lathliss, Dragon Queen EDH deck, and just last night was at my LGS for some casual play.
My thought process was, if I activate Lathliss' ability to give my Dragon creatures +1/+0 then when Scourge of Valkas enters the battlefield it would enter as a 5/4, and therefore trigger Where Ancients Tread's ability.
The table seemed to conclude Lathliss' ability only applied the buff to Dragon creatures I currently controlled on the battlefield.
I'm confused, because it appears Lathliss' ability would apply to all Dragons, much the same as having Honor of the Pure on the field while casting a white creature with 2/2 would not cause Mentor of the Meek to trigger because the creature entering would actually be a 3/3, due to the added buff from Honor.
Could I have some clarification on this? Was my table right in that the interaction was not valid?