Completely false. If tomorrow Russia defended Syria against Turkey by attacking mainland Turkey, Article 5 would certainly apply. Similarly if a Libyan faction fired a ballistic missile onto Paris tomorrow I guarantee Article 5 would be considered. Or if Iran sunk a Turkish ship off the coast of Syria in retaliation for a strike on the SAA, etc...
NATO means that Turkey doesn't have to worry about any retaliation from Syria's allies. That is to say, impunity. On the other hand if Israel invaded Syria and IRGC troops fired a missile onto a base in Israeli territory, the US wouldn't invade Iran, but they would if it was into Turkey. That's a significant chilling effect.
Preventing allies from retaliating