I'm on mobile so I must be brief sadly.
1. The animation matches Force Drain.
2. Force Sever is known to Jedi and is non lethal. Also, it is not infallible. Exar Kun resisted Odan-Urr's attempt even though the latter pretty much mastered the technique during the Great Hyperspace Wars.
3. In a Dark Side Exile playthru, she explicitly feeds on the Jedi Masters one at a time using the same animation and gains power in doing so.
In light of this, Gigadrain is not Force Sever, and furthermore you have not established any reason why this trio would succeed where the council members failed. Hell, Kavar was able to Force stun an entire cantina while on Onderon, arguably a Dark Side nexus thanks to the Krath. They were not weak mooks with high titles.