At the M&B Modding Discord cokjan showed two months ago some tests which he did to prevent the facehugging. His idea was to (copy pasting his words)
To summarise it for you: It's not done via an animation (seems not to be possible that way) but a trigger within a mission template which requires scripting.
After being asked "So the spear animation forces a leg animation with displace position or whatever that flag was?" he responded with."controll[...] agent footwork so they don't facehug like a dumb person via animations (didn't use set scripted destination because i think it would require 2 try_for_agents?)
nope, it still uses a mission trigger to set the animations. but that could be an alternate solution for removing the need for try_for_agent by adding a displace pos flag for blocked/parried animation. albeit making it kinda janky for player and ai movement
To summarise it for you: It's not done via an animation (seems not to be possible that way) but a trigger within a mission template which requires scripting.