Comment by nayroclade

As I understand it, funding was provided by NLnet[1], a longstanding Dutch non-profit that focuses on supporting open internet technologies. The funding was provided specifically for reviving Servo. By the looks of it, the money itself mostly comes from the EU, which has various grant programmes to fund open access technology, digital sovereignty, etc. Given several Servo contributors worked for Igalia, I expect they submitted a proposal to NLnet for them to fund Servo development, and it was successful.

[1] https://nlnet.nl/project/Servo/