I think that sub_field_1 and sub_field_2 in that example are supposed to represent the field names of the sub fields, but using the field keys is always the better option when using update_field(). The second example in the same code for flex fields better illustrates the way the value array needs to be for repeaters, with the exception of the acf_fc_layout index. Repeaters and flex fields work the same way with the exception of the layout name.
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