I think it is the symbolism of the Risen Christ.
Here is a link:
This quote from the link you sent is very helpful to my understanding:
In the service of Proskomide, the priest blesses it and says, “In remembrance of our Lord and God and Savior, Jesus Christ.” We must then ask, was Christ not full of the Kingdom of Heaven? Was He not full of the Spirit and good works to which the Fathers liken leaven? It does not appear entirely inappropriate that we should commemorate the Body of Christ with leavened bread, so long as it is not sourdough or made of coarse and cheap flour.
And that the Savior received first-fruits of those whom He was to save, Paul declared when he said, ‘And if the first-fruits be holy, the lump is also holy,’ teaching that the expression ‘first-fruits’ denoted that which is spiritual, but that ‘the lump’ meant us, that is, the animal Church, the lump of which they say He assumed, and blended it with Himself, inasmuch as He is ‘the leaven.’
-St. Irenaeus, Against Heresies , Book 1, ch. 8, para. 3
If the bread and wine we offer are a sacrifice, then they can only be likened to the first-fruit sacrifice of the Old Testament, since Christ’s death replaced all other atonement for sin. And, as we recall, the first-fruit sacrifice was made with leavened bread. This is what St. Irenaeus is implying by his mentioning of the first-fruits. We offer ourselves with the bread (i.e. the lump as the Church), but we are filled with Christ (i.e. as leaven). We cannot offer ourselves apart from Christ as an unleavened loaf, and so we use a leavened loaf to symbolize Christ within us as we offer the spiritual first-fruits of our lives."
From this, it appears that leaven is used many different ways in the Scripture to symbolize many different things, and that the unleavened bread used as a sacrifice for sin in the old testament, prefiguring the coming sacrifice of Christ, might be understood as our attempt to offer sacrifice on our own without Christ, but when Christ's light fills our humanity (which He did through His incarnation, life, death, and resurrection) as leaven fills the bread) it makes it new and our sacrifice thus capable of truly pleasing God. It seems a hard and complex issue and I look forward to reading and studying further about this.