This is not the ONLY argument you know. Read again the post you quoted.
Then the tree would have died instantly after this.There are other explanations for that (such as, you know... Kuja destroying pretty much all of Terra,
In the Crystal World, Kuja says that the process of assimilation between Gaia and Terra already started(by the tree). It don't need Garland or the Genomes to help it.or the fact that Garland and the Genomes' are unable to monitor and help the flow of souls from one world to the other).
And i still want to know why some people say Necron retreats, after being defeated. It seemed to me that he was... blowing up, exploding, being completely and utterly destroyed.