this is actually false
The issue here is somehow different as if you ever join in a game that was running allready a node can be purified even without a single thrall is killed. It is possible though you come in time and there are a few Thralls left you can kill and the node will not be purified too.
It might be possible that the purifycation depands on more than a single circumstance like how long you are in a mission and how many thralls were killed allready.
Another problem here is how the thralls actually spawn which is based on a few parameters:
Enemy spawns: more thralls are spawning when there is no actual limit in spawnnumbers for excample during alerts or during a survive or interception mission.
Progress: If you make progress on youre mission thralls often spawn directly this is best seen in spy missions as the moment you enter a spy area a thrall will spawn as long as the maximum limit is not reached. the same effect you gain with milestones while heading to the exit while coming closer to the exit a thrall is spawned.
Time: this one is actually the worst mechanic and it works like this. Whenever you have aquired the progress parameters of a mission and you are allready near the exit but you have possible thrallspawns left you can wait for others to appear, There is a time parameter involved which spawns additional thralls after a few minutes in the mission thats why often thralls spawn when some are allready at the exit and a teammember is behind.
So for purification research you have to considder those 3 parameters as well as the maximum spawn numbers per mission which seems to be 10 possible thrall spawns. It might be possible that whenever you reach those 10 spawns in total the purrification of the node also occurs while having less than a specific percentage of thrall spawns lead to no purrification. For a late entered game this would also explain how it works as it might count all thrallspawns of the actual game in account so if you enter a game 80% of the possible spawns are gone allready you end up with a clean node while a node with only 50% of the possible spawns might stil work for you.