First, this is not a security but a safety concern.

Second, this should have been reported by OP to a FA immediately. Even if it was a critical part, that doesn't mean that the aircraft will fail. Simply that it is unsafe to fly. While it is primarily F9's responsibility to deal with the issue, someone seated inside the aircraft is at risk as well. If reported, the Captain can make a decision.

Third, clearly the Captain should have known of this and apparently did not. The fact that ground crew are taking photos suggests that they know that something is missing. Many such issues are not a reason to hold the flight, but nonetheless it is the Captain's final decision.

Finally, writing to F9's customer service people is pointless. That is a perhaps useful channel to complain about the lukewarm coffee and the like. But, the low-level people who choose cut & paste answers have a stock response for this stuff and that is what OP got. Send a complaint to FAA. That will generate an inquiry to F9 for full documentation which will include a paper trail showing (or not) that a decision was made by the Captain.