I would say someone has got it wrong somewhere along the line.
The International FA Board said
"Decision 1
Players must not reveal undergarments showing slogans or advertising. The
basic compulsory equipment must not have any political, religious or personal
statements.
A player removing his jersey or shirt to reveal slogans or advertising will be
sanctioned by the competition organiser. The team of a player whose basic
compulsory equipment has political, religious or personal slogans or statements
will be sanctioned by the competition organiser or by FIFA."
Law 4 defines compulsory equipment as follows
"The basic compulsory equipment of a player comprises the following separate
items:
• a jersey or shirt with sleeves – if undergarments are worn, the colour of the
sleeve must be the same main colour as the sleeve of the jersey or shirt.
• shorts – if undershorts are worn, they must be of the same main colour as
the shorts
• stockings
• shinguards
• footwear"
Nowhere is headwear defined as "basic compulsory equipment" so why has the hijab been banned?