I actually flew the Air Sinai flight once, o/w from CAI to TLV in 2018. Yes, I had to buy flights (in cash!) from a strange retro Egypt Air office in a Tel Aviv office building. Others had told me that the Cairo airport hides this flight and it isn’t easy to check in or find the gate but I found it listed normally. I believe there was only minimal extra security to board. The load was extremely light - less than 20. There were many FAs and I believe even a crew deadheading. While the plane was unmarked, the interior and uniforms were normal Egypt Air. Also, the flight was extremely short, maybe 45 mins, and they served a full meal! Deplaning at TLV was also completely normal. Glad I got to try this once. Happy to answer more questions if I remember more details.