That appears to be a foul by the defender in white, or at the very least an offsetting foul. This should NOT be considered to be a block and a turnover.
12.8. Every player is entitled to occupy any position on the field not occupied by any opposing player, provided that they do not initiate contact in taking such a position.
Initiate contact: Any movement towards a legally positioned opponent (either their stationary position, or their expected position based on their established speed and direction), that results in unavoidable non-incidental contact.
12.9. All players must attempt to avoid contact with other players, and there is no situation where a player may justify initiating contact. “Making a play for the disc” is not a valid excuse for initiating contact with other players.
17.1. Dangerous Play:
17.1.1. Reckless disregard for the safety of fellow players is considered dangerous play and is to be treated as a foul, regardless of whether or when contact occurs. This rule is not superseded by any other rule.
17.2.1. A Defensive Receiving Foul occurs when a defender initiates contact with a receiver before, or during, an attempt to catch the disc.
17.1.1. and 17.2.1. make it clear that it is not relevant that the defender got the disc before the contact occurred, as the contact occurred during an attempt to catch the disc and also appears to constitute dangerous play.
From the footage it appears that the defender is the one who initiates the contact and therefore they are responsible for causing the collision. However it could be possible that both players initiated the contact which would be an offsetting foul:
17.10. Offsetting Fouls:
17.10.1. If fouls are called by offensive and defensive players on the same play, the disc returns to the thrower.
17.10.2. Non incidental contact that occurs as two or more opposing players move towards a single point simultaneously should be treated as offsetting fouls.
Important note: At its basic level, Ultimate is a non-contact sport that has a bias towards the offense. In these sorts of incidents where contact occurs as two players converge on the same spot, the majority of outcomes will result in the offense retaining possession (either via the receiver or the disc being returned to the thrower). If this sort of collision did result in a turnover, then the rules would be rewarding dangerous play and would be in direct contradiction of one of the basic rules - that Ultimate is a non contact sport.