I think the sign is directing people coming up from Brixton, rather than those coming from Stockwell, who would already be on the stepped path. It's almost imperceptible, even when you're riding past it but the transition from black road to blue cycle patch is flat on the south of the sign, but stepped (dangerously so) to the north of it. If you miss the sign and try to join the blue after the sign, you will get hurt.
I think the sign is directing people coming up from Brixton, rather than those coming from Stockwell, who would already be on the stepped path. It's almost imperceptible, even when you're riding past it but the transition from black road to blue cycle patch is flat on the south of the sign, but stepped (dangerously so) to the north of it. If you miss the sign and try to join the blue after the sign, you will get hurt.