It depends on the third party and who is paying them to investigate. If coinbase is the one paying, the third party is unlikely to be unbiased.
You don't pay a third party to find damaging stuff about your company. Do you really think this third party would ever find something that made Coinbase look bad?
Arthur Andersen taught us that two decades ago.
> three of these people filed complaints during their time at Coinbase. All of those complaints were thoroughly investigated, one through an internal investigation and two by separate third-party investigators
Using third parties sounds like a good idea here.