Well, sort of. They have their idea of what they want (approximately "security for the masses"), and if your pull request doesn't go in the same general direction they don't care about it. If it does, they're positivish but they still aren't eager about fixing your code.
FWIW I've had one (very small thing) accepted and another not. That second wasn't a net positive if "security for the masses" is the only important goal.
I fixed handling of multiple SIM cards on MediaTek's forked Andoid, which had added multiple SIM support before Android itself did. There were bugs in the MediaTek code but Signal could be made to work. However, the code I wrote to make Signal work looked risky; it seemed possible that it might break something on other phones.
FWIW I've had one (very small thing) accepted and another not. That second wasn't a net positive if "security for the masses" is the only important goal.