FB are large enough that they do a ton of their own very low level work - up to and including things like their own bespoke datacenter switches and SDN platform, stuff which is almost wholly distinct from what their core product is. (https://engineering.fb.com/2019/03/14/data-center-engineerin...)

It seems entirely possible that, even putting aside the PR, they're concerned about making sure a robust modern systems language stays around/thvives so that they have a robust modern systems language to use.