No, this is wrong, Severson shouldn't stick with McDavid, that's Greene's job here. McDavid and Lucic are very close together, where Greene can play the 2on1 with them to prevent a pass back to McDavid from Lucic, while sticking close enough to Lucic to keep him from going in for a deke. Lucic can either shoot or pass to Draisaitl based on Greene's positioning here. Greene plays this perfectly, and Severson blows his coverage.
@7:18 Greene and Severson are both positioned perfectly here. McDavid has the puck in the middle with Lucic to his left and Draisaitl to his right. McDavid can't really go between them, and Greene/Severson are in a good position to cover either player on the wing should McDavid make a pass.
@7:20 McDavid has stayed in the middle but cut slightly to his left, and slid a pass over to Lucic. Greene is in a perfect position to play the 2on1 here and either have Lucic try and force a pass to McDavid, who Greene is on, or take a shot on goal that Kinkaid should stop based on the angle and distance. At this point, Severson needs to cover the third option, the cross ice pass.
@7:21 Lucic passes to a wide open Draisaitl. Severson has his back to the pass and is doing absolutely nothing to contribute to defending this play. He's "covering" McDavid, as you can see from the image is already covered by his partner in Greene.
The entire job of a defenseman on an odd man rush is to prevent a clean cross ice pass or a deke on the goalie, as you severely limit your goalie's ability to make a save if that pass gets across. Severson blows his job here and leaves the most dangerous play wide open to cover a man who wasn't in a dangerous position and was already covered. McDavid does nothing special on this 3on2, and it's completely misleading to claim "oh it's McDavid what can you do?" when this was a textbook breakdown of defensive coverage. Teams run overlap plays and such specifically to draw defensemen out of position to open up passing and shooting lanes, but the reason they all just stayed in a straight line here is because there was no need to draw anyone out of position, Severson did it for them.