Just tried it on my Android tablet for the MotoGP replay and the cast option is there for me. What device are you trying it on?
That said when I tried last night with the TV (and thus the dongle) turned off there was no cast option. But I also noticed the MotoGP replay video was using the enhanced player without the option I could see of using the standard player and you can't cast from the enhanced player. Today it was using the standard player.
I don't like the way the enhanced player flags up spoilers (goals, cards wickets etc) on the play bar and wish it did not do that.
I did briefly use the IOS (iPad) and Android app on the Sunday evening c 9:50pm and both gave me options to cast using Chromecast and did so successfully for that day's MotoGP race.