Why are they believable? Russia's ability to project power is at risk.
Tartus in Syria is Russia's only foreign naval port.
Support for Syria is a proxy for support for Iran.
If Iran should shift to a pro-Western stance, the great oil fields of Turkmenistan and Kazakhstan will send their oil south through Iran to the rest of the world.
As things stand, these countries depend on Russian pipelines for their national income. Russia’s gas and oil leverage over Turkey, Ukraine and much of Europe would evaporate.