Huh, I thought it was always defined by where the outside guys on offense (TE) were lining up.
It's like people that say they are going to get a "coke" when they actually drink Pepsi/Dr. Pepper etc..
Marinelli like many coaches usually calls them Will, Mike and Sam.
Some d-Coordinators call their LBs by other names
Buddy Ryan called his outside LBs Jack and Charlie in the 46 defense.
Some 3-4 DCs call their inside LBs Mike and Will but since there are only 2 of them there really isn't a "Middle" LB.
A 4-3 in Nickle only has 2 LBs which means neither is really in the "Middle" but they're generally referenced as MLB/WLB or Mike/Will.
It's all just naming conventions. The specific duties of players varies from scheme to scheme.
Mixing up the duties of defensive players is a good way to deceive offenses. If all 3 could play all 3 positions then it would be hard for QBs and blockers to make pre-snap reads.
I think we'll see some snaps with Jaylon lined up on or near the line with the option to rush the passer despite him being the Mike LB.