Because that “CharSequence contains only Unicode digits” (quoting your linked documentation).
All of the characters return true for Character.isDigit:
Some Unicode character ranges that contain digits:
- ‘\u0030’ through ‘\u0039’, ISO-LATIN-1 digits (‘0’ through ‘9’)
- ‘\u0660’ through ‘\u0669’, Arabic-Indic digits
- ‘\u06F0’ through ‘\u06F9’, Extended Arabic-Indic digits
- ‘\u0966’ through ‘\u096F’, Devanagari digits
- ‘\uFF10’ through ‘\uFF19’, Fullwidth digits
Many other character ranges contain digits as well.
१२३ are Devanagari digits:
१is DEVANAGARI DIGIT ONE,\u0967२is DEVANAGARI DIGIT TWO,\u0968३is DEVANAGARI DIGIT THREE,\u0969