Yeah, looking at this play it's clear McDavid should have been penalized first but the refs decided to let them play on and therefore couldn't call either guy.
Garland pokechecks the puck off McDavid and is likely going to get an empty net goal. McDavid realizes that and basically jumps on him, which is interference/holding. Garland reacts to that by holding McDavid down. The play basically continues until McDavid gets up and crosschecks Garland to the head.
The first penalty in that play was clearly McDavid. The refs didn't want to call it and let them play on so Garland took advantage and held him down.