If I put Empoleon onto my Bench using its "Emergency Surfacing" Ability, does Torterra's "Evopress" attack add 50 to its damage for the unevolved Empoleon?
Yes, even though Empoleon doesn't have any lower stages attached to it, it is still considered an Evolution Pokemon.
Source: Brilliant Stars FAQ; TPCi Rules Team (2022-02-24)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH9_37
SWSH9_37
SWSH9_8
SWSH9_8