Hi,
The reason could be using the same ECAM abbreviation "ALTN BRK" for 2 alternate braking systems:
Alternate braking with antiskid
Alternate braking without antiskid (brakes being supplied by accumulator only)
AND, depending on A/c's MSN, some other differences, given the presence of ABCU or not.
In fact going through the FCOM DSC-32-30-10 (landing gear section) you can see a description how normal and alternate braking, with or without antiskid, work.
To answer your question, I would sah that a B+Y hyd failure doesn't prevent the use of normal braking, being the main and only method of braking; even if it's still a backup, there is nothing you can do to use the accumulator, hence the alternate braking without antiskid, unless you depresurize the Green system too or switch OFF the A/SKID & N/W STRG.
With a G+Y hyd failure you lose both normal and alternate braking with antiskid; the remaining one is the accumulator, or alternate braking without antiskid; if you look at the status page (or FCOM PRO-ABN-29) it's specified you can brake with the accumulator, hence with an alternate braking without antiskid. To put it inoperative it would be a nonsense..
Just my 2 cents, hope it can help.
Ciao!
Moto