I don't think that's what it is, plump.
Forgive me, for I only learned about indirect discrimination 5 minutes ago so this is very much me processing what I've just read. But it's really interesting.
Indirect discrimination is treating two people in the same way, but by doing so, by choosing a particular criteria for all, you can actually discriminate against one group.
One example I've just read (illustrative and not meant to be an ACTUAL example) is to set a criteria that all applicants must be over 6ft tall. It is applied to everyone, but by its very nature it prohibits most women from applying and is therefore sex discrimination. Even though it is not treating either gender differently.
This is a new thing for me to learn about 
So the argument - if I am understanding correctly - is that not wanting to employ someone who has been a SAHP, on the grounds that their skills are not uptodate, is discriminating against women because most SAHP are women.
I must say, it's something I had not really understood.