I think a point you aren't emphasizing enough is that Belgorod is less than 80km from Kharkov and the PAC-2 has a range of >160km... So, Belgorod was already under Ukraine's AD umbrella. It doesn't help that the IL-76 isn't stealthy, has no active defence EW capability, and is literally a giant flying boat. It also doesn't help that Belgorod is connected along the South Eastern Railway to Voronezh, which is a major logistics hub in the region. There is no reason an IL-76 should be flying over Belgorod without clearing with Ukrainian AD. Hell, Belgorod is so close to Kharkov that you wouldn't even need a Patriot system: even the IRIS-T system (range 40km+) would be enough given how static the frontline has been there. Belgorod sits in a cauldron.

Everyone is so convinced Russia must be lying that they're not considering what would logically make sense.