I agree that coordinating reimbursement via a Signal channel would be very dumb but that's exactly what the indictment says they did. I'd encourage you to read pages 15-18. I've bolded some of the relevant selections below for you. The remaining question mark is whether GAP leadership was in the Signal group.
From at least in or around March 2022, SAMUEL BANKMAN-FRIED, a/k/a "SBF," the defendant, and his co-conspirators began coordinating political contributions paid for using FTX and Alameda funds through an encrypted, auto-deleting Signal chat called "Donation Processing." From time to time, BANKMAN-FRIED and his co-conspirators substituted other individuals in BANKMAN-FRIED's place for contributions originally intended to be made in BANKMAN-FRIED's name. For instance, shortly before the midterm elections, an FTX employee was directed to "wire $107k from [BANKMAN-FRIED] personal to New York State Democratic Committee," but then was asked by BANKMAN-FRIED to "update this to a 107k contribution from [CC-1]."
Notwithstanding his awareness of the campaign finance laws, in order to conceal the true source of the funds, SAMUEL 16 Case 1:22-cr-00673-LAK Document 80 Filed 02/23/23 Page 16 of 39 BANKMAN-FRIED, a/k/a "SBF," the defendant, agreed with others that funds for contributions would be transferred from Alameda's bank accounts, which also contained FTX customer funds, to bank accounts in the name of the donors, and then quickly transferred from those individuals' bank accounts to political campaigns.
In total, SAMUEL BANKMAN-FRIED, a/k/a "SBF," the defendant, and his coconspirators made over 300 political contributions, totaling tens of millions of dollars, that were unlawful because they were made in the name of a straw donor or paid for with corporate funds. In dozens of instances, BANKMAN-FRIED's use of straw donors allowed him to evade contribution limits on individual donations to candidates to whom he had already donated.
Instead, an internal Alameda spreadsheet noted over $100 million in political contributions, even though FEC records reflect no political contributions by Alameda for the 2022 midterm elections to candidates or P ACs.