No, based on the rulebook context, the phrase "in this cave" refers specifically to the row in which the dragon is located (either the Crimson Cavern, Golden Grotto, or Amethyst Abyss) and does not depend on when spaces were excavated during the round. The rulebook explicitly states that "in this cave" abilities apply to the specific row, not the entire player mat or the timing of excavation.

This interpretation is supported by the explanation on page 11, which clarifies:

• "IN THIS CAVE": If a dragon’s ability has the text “in this cave” (e.g., Gain 3 for each other dragon in this cave.), it’s referring to the specific row in which the dragon is located (either the Crimson Cavern, Golden Grotto, or Amethyst Abyss), not your entire player mat.

Thus, any dragons added to the same row (cave) later in the round would still count for abilities specifying "in this cave", as long as they are in the same row. Timing of excavation does not affect this rule.

Source: Wyrmspan Rulebook (pp. 1, 11, 13, 6)