Actually, the tunnel visible on streetview is also visible on old mapy.cz aerial photos, and seems like it could go feed into the pond inlet. That could mean that this might indeed be the tunnel for excess water from the Blanka tunnel complex with the possibility to divert/pump water into the waterplay. There also seems to be another tunnel running parallel to this one ~10 meters to the north from it. Because this one was still being laid down when the aerial photos were taken it might be that it is just the permanent version that wasn't yet completed then.

It is definitely connected somewhere, but not sure how. If it were connected to the pond inlet tunnel it would need to be longer and possibly gain elevation so that seems unlikely. If, however, it were connected to the outlet it wouldn't flow into the pond (which would conflict with info from [1]), unless there is a more complex arrangement there. On google streetview from 2012 a water tunnel of some kind can be seen exposed roughly where the waterplay is, which seems to lead to where the current pond outlet is. This exposed tunnel might either be a temporary structure or a bypass tunnel for excess water that isn't pumped into the waterplay (the existence of which I've seen mentioned somewhere but don't really remember where rn).

[1]: https://www.praha-priroda.cz/vodni-plochy-a-potoky/studanky-dle-katastru/hradcany/u-sanci/#:~:text=Do%20rybn%C3%ADka%20p%C5%99it%C3%A9k%C3%A1,do%20jeho%20obtoku.