I can’t imagine they are, I’d be surprised if they are all eligible
I've read somewhere that any U21 players is eligible, but if that's the case I don't know why they have this list on the UEFA site. This is our registered squad list for 2020.
, Adrian, Kelleher, Jaros
Defenders: Virgil, Gomez
, Trent, Robbo, Matip, Tsimikas, Rhys Williams, Neco Williams, Billy Koumetio, Yasser Larouci
Midfield: Fabinho, Gini, Thiago, Milner,
Keita, Hendo, Ox, Curtis Jones, Shaqiri, Elijah Dixon-Bonner,
Jake Cain, Leighton Clarkson
Forwards: Salah, Jota, Mane, Bobby, Minamino, Origi, Harvey Elliott
, Layton Stewart, Liam Millar, Luis Longstaff, Paul Glatzel
The players in bold do not appear to be present in Denmark. Elliott obviously is no longer with the team.