I think the disconnect here is that Google engineers are much more likely to answer the low volume of technical "nerd-snipey" questions from other developers than the high volume of non-technical questions they'd get from the general public for something like Gmail.
By my thinking, that's a "public" mailing list. They're not hiding it from you. The opposite, really — they're trying to get everyone to know and use it, by making it free to any GCP customer, while the actual CSR kind of support requires paying for a subscription to a higher support tier. The mailing list, presented in Google Groups format, is literally what GCP calls their "support forum." It's supposed to take on all comers, including dumb customer asks.