Ok.
First question, (and this isn’t a “if you haven’t played the game at a high level you don’t know ****” post), but have you played defense? Just asking because that short light crosscheck has been a staple of D for years. The reason is because it’s highly effective at putting a guy off balance so he cannot handle the puck.
Eichel plays the role the D normally does and gives McD a light shove to the small of the back which puts him off balance and into the boards. The reason I know this wasn’t a dive was because when a player “dives” he will typically sprawl and lay there, WITHOUT contacting the boards with his head, which is definitely not a good idea.
The thing I think you need to get past is realizing that it’s not always about how hard you hit someone, but where and when. In this case McD was focused on the puck, had no idea the shove was coming and was only a couple of feet from the boards (which is extremely dangerous). Eichel absolutely should have gone for 5 in that instance. Maybe it would have deterred him from doing similar to Fischer...or not?