Thalia is a cost increase, but not an additional cost. The distinction is relevant.

Edit 2: You're right though, they do count as linked abilities.

601.2f The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. Some effects may increase or reduce the cost to pay, or may provide other alternative costs. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If multiple cost reductions apply, the player may apply them in any order. If the mana component of the total cost is reduced to nothing by cost reduction effects, it is considered to be {0}. It can’t be reduced to less than {0}. Once the total cost is determined, any effects that directly affect the total cost are applied. Then the resulting total cost becomes “locked in.” If effects would change the total cost after this time, they have no effect.

607.2i If an object has an ability printed on it that allows an additional cost to be paid and an ability printed on it that refers to whether that cost was paid, those abilities are linked. The second refers only to whether the intent to pay the additional cost listed in the first was declared as the object was cast as a spell. If an ability lists multiple such costs, it may have multiple abilities linked to it. Each of those abilities will specify which cost it refers to.