Not to mention 1-second GOPs are ludicrous for a broadcast/ondemand publisher like the BBC.

However, I think libaom performance was the deciding factor for this. They say right there that they chose this arrangement because it would allow them to deliver 10s chunks with reasonable latency.

With SVT-AV1 and appropriate hardware, there's no reason they couldn't do 10s like they should, and in that case AV1 would probably shine.