Bedrock Tortoise's ability doesn't actually change any creature's power. It changes only the amount of combat damage the creature assigns. All other rules and effects that check power or toughness use the real values, even if they cause damage "equal to a creature's power" to be dealt.
During your turn, creatures you control have hexproof.
Each creature you control with toughness greater than its power assigns combat damage equal to its toughness rather than its power.
