I very much doubt anyone believes any "brown envelopes" were actually passed to the FAA.
I expect that to be an exaggeration of excessive allowance expenditure for FAA visitors allowed by Boeing, and the secondary envelope being lucrative jobs post FAA employment at Boeing.
Many favours would still be owed to the regulators staff, if that is the case.
Could the FAA step aside and authorise EASA to re-certify the MAX? - to prove complete transparency.