Science-first is an approach to claims anchored entirely in the material world.
Empirical claims become truth through science. Speculative scientific ideas may be explored, but they are not treated as true until adequately supported. For example, string theory is speculative. It dreams of 10, 11, or 26 realms of existence. In a science-first approach, it remains false until proven. In science-first approaches, Valhalla, Santa Claus, or supernatural beings may also be discussed or imagined, but they are not true because they lack scientific support strong enough to move them into our shared understanding of reality.
Science-first does not forbid speculation. It forbids speculation from masquerading as truth.