I do work for a SolarWinds Customer. SolarWinds told us on Thursday that the certificate was going to be revoked on the 21st. Then yesterday they told us the certificate wasn't being revoked until February 2021.
This says to me that the certificate itself probably wasn't compromised. The attacker must have found a place in the CI pipeline where they could insert code and get it signed automatically.
I'd be surprised if signing was done automatically, that would be really bad. More likely it was done manually on a package that came out their build system, without anyone stroking their beard to wonder if that system had had its compiler replaced or its cache of dependencies poisoned.
This says to me that the certificate itself probably wasn't compromised. The attacker must have found a place in the CI pipeline where they could insert code and get it signed automatically.