This is helpful thank you! I just wonder if this does not in any way mean the policy is any less morally (and potentially legally) discriminatory.
First, just because they must have thought through it does not mean they did that well right? Take
Freshfields which has paid a huge financial and legal cost for giving what was later deemed illegal advice on taxes in Germany. I am sure they thought through it but it turns out they were wrong. To look at a non-legal example McKinsey is paying a huge financial and PR cost for giving bad advice on opioids in the US. So, just because it is a big firm and it must have thought through it does not mean they came to the right decision.
On the point of whether they found it a useful way of picking candidates, again this is no excuse. Say you run an analysis and find that BAME applicants get along less well with what is still a predominantly white c-suite executive group (Note: this is just a hypothetical example to make the point and a real analysis would probably disprove this given things are, thankfully, changing) and use that as an excuse for hiring fewer BAME candidates, would this be OK even if the impact is better PEP? Actually finding random typos IS a better way because that is some sort of measure of attention for detail but age is not.
Again, I do appreciate where you are coming from but I really think saying ahh they are a big firm and must have thought of it does not mean they might not have got it wrong and it being potentially useful to discriminate does not make it OK.