I think Antrim are playing an u16 squad in the competition. Antrim are a "graded county" therefore minor panelists are not eligible to play in the Celtic Challenge.
The following is taken from the Operations Manual:
Involvement in the County Minor Hurling Panel
Players who are on the Inter-County Minor Hurling Panel in the following Counties are not eligible to participate in this competition: Antrim; Down; Carlow; Clare; Cork; Dublin; Galway; Kilkenny; Kildare; Laois; Limerick; Meath; Offaly; Tipperary; Waterford; Westmeath; Wexford.
A player will be deemed to be on the Inter-County Minor Hurling Panel if he is one of the 24 named match day players for the Provincial Minor Hurling Championships.