First of all, clearly Github can't answer for the developer how their bot's token was compromised, that's something the developer needs to find out. Instead they are repeating this statement like it's out of their hands.
But more concerningly, I don't believe the explanation is supported by the Github history which says the compromised commit was "authored" by Renovate and "pushed" by @jackton1. It's obvious how the first part was spoofed, but the second part is concerning as it indicates the @jackton1 account was compromised not @tj-actions-bot. If I'm missing something please let me know.