From what many people assume, Solid Dragon's effect can't activate during the damage step because it doesn't directly involve stats modification. For example against a Gravekeeper's Guard.
Then, can Slash dragon's effect protect from this same target effect ?
It is actually because no one can summon a monster during a damage step. Damage step restrict solid dragon effect activation in the first place, which explain why he cant use its effect since its involves special summoning
Special Summon can happen during Damage Step, if a card's effect that Special Summon specifies its activation window as something only happens during Damage Step.