This video was posted yesterday. 4/26/22
Turkey has announced its closure of Turkish airspace to any Russian commercial or military aircraft trying to re-supply its forces with either more troops or munitions in Syria. So the FSB whistleblower's predictions and concerns about the plight of Russian soldiers and the Russian military posture in Syria appears to have gotten worse than even he originally thought.
Simply put, any Russian warships currently in the Black Sea that do not have a home port there, will not be allowed back into the Black Sea if they ever leave (even briefly) during the Ukrainian war for any reason. No Russian warships in the Mediterranean can get into the Black Sea unless they can prove they have a home port there. Which I think means that zero ships of the Russian fleet currently in the Med will be able to reinforce the current Russian Black Sea fleet. If any of those ships in the Med have a home port on the Black Sea, they probably would have already been brought in. The Russians can't bring in another ship from elsewhere to replace the sunken Moskov. The same is true for any more warships they might lose in the Black Sea during this war. They can't be replaced. They're just left with a weaker fleet. Which is possibly very good news for the Ukrainian port city of Odessa.
If a Russian warship in the Black Sea tries (using the cheapest and most practical method) to ferry supplies directly from a Russian Black Sea port down to Syria, the ship won't be allowed back in. The FSB guy, (OP) was worried that the only way to re-supply the Russian forces in Syria would be to ferry everything down by plane. (Which is extremely expensive and impractical for some military hardware.) Now Turkey is saying that those flights would not be allowed over its airspace either. Not even troop transport planes to rotate forces in and out. Again, Turkey is a member of NATO. I can't imagine that any other NATO members like Bulgaria, Romania, or Greece would allow Russia to fly military aircraft over its airspace either down to or from Syria either.
Possibly Iran might. Through Georgia, Armenia or Azerbaijan if they agree. Or over the Caspian Sea, over Iranian airspace (maybe), through Iraqi airspace (maybe) and through the easternmost part of
Syrian Thomas airspace. Then they would have to fly the entire width of Syria to reach the westernmost
Syrian Thomas Mediterranean port city of Tartus where the bulk of Russia's military contingent is based from. Any of those scenarios are long and costly trips. So Russia's fastest, cheapest and closest air corridors down to Syria are pretty much blocked off right now.
Also though as OP stated earlier, Russia does not have "extra" military supplies and manpower lying around right now (to re-supply Syria) because the bulk of their troops, food rations, munitions, fighter aircraft, spare parts, weapons, transport vehicles, etc, are being used
in or being shifted
to the war, "Special Military Operation" in Ukraine.
And as I've said before, in spite of its glory days of the Soviet empire...Russia today is
not a rich country. It can't afford to fight two different conflicts in different places for long. (Even if they had a really good and competent military, which is questionable right now.) If any real fighting flares up again soon in Syria, they will be in big trouble. Because Russia's economy can't sustain the cost.