This statement was somewhat strange in my opinion.
The Russians blame the Americans for the recent advances of Jaysh Al-Fath (JaF). They say that they wanted to strike JaF but they did not do it because the Americans asked them not to.
So the Russians knew in advance that JaF would screw Assad, but they did nothing to prevent it, and the reason is… an American request!
Perhaps what the Russians say is true, but it is also possible that they did nothing against JaF because there was nothing they could do to save Assad from defeat.
JaF has proven over and over again that it can defeat Assad. This organization managed to kick Assad out of Idlib city and the entire Idlib province, and it managed to take Abu Ad-Duhour airbase. Assad’s air force and firepower were unable to prevent those grave losses.
Many people (including myself) believe that the defeats of Assad in front of JaF could have been the reason behind the Russian military intervention in Syria. Before the Russian intervention, JaF was invading Hama province. Back then I discussed the possibility that JaF would reach Rastan and revive the uprising in Homs.
JaF seems to have many thousands of fighters, some of whom are hardline jihadists who use suicidal battle tactics (عمليات انغماسية). What can a regular army do against an invasion by many thousands of fighters led by suicider jihadists? I am not a military person, but I think that stopping such an invasion would require big firepower which Assad obviously does not have. Otherwise he would not have been defeated so many times by JaF.
JaF has almost encircled Aleppo. If they muster a big force and launch a big attack on Ramouseh from their current positions in Khan Touman, there is a good chance that they will besiege Aleppo. This explains the Russian anxiety. The Russians want the Americans to prevent such an attack.