They used to interview using the kind of brainteasers found in books like the ones Mensa used to make. The algorithms approach, I suspect, is just a CS proxy for an IQ test just like their old approach was. It would also filter for youth, which they semi-openly advertise as well (see chess literature on brain age for what I mean).
This is because it's actually not allowed to use IQ tests as a screening mechanism for most jobs (you need a valid reason and "software engineer" probably isn't good enough to justify the liability). By using algo questions they can select for something that might correlate very well with IQ, that also makes sure the candidate has real coding knowledge, and which has much less liability
Conformance too (due to the prep time)