Goldman Sachs is more like a collection of companies (divisions) than one company. Further, there is often a Chinese Wall between divisions preventing them from having access to one another to prevent insider trading.
Goldman Sachs will look stupid if they advise against a sale at $54 if they also advise to sell and believe the price will be $30.
Twitter board is not buying GS opinion. It's clearly worthless if one guy says "Twitter is worth $30" and the other guy says "Twitter is worth more than $54".
They are paying GS to tell them what they want to tell the public i.e. don't sell to Musk.
Twitter board is just too incompetent to execute such cover up. I assume there's at least one bank they could hire that has buy rating on Twitter to justify their "don't sell" advice.