Fang Zheng had two reasons for making such a speculation.
First, based on the structure of the virus that Minamoto Sakura and the others were infected with, there had been no fundamental changes in the structure of these viruses ten years ago. But today, ten years later, the virus infected by Walnut and Rea has undergone significant changes. Of course, you could say that the virus might evolve due to environmental mutations, but it becomes problematic when the original functional structure is replaced. Even if evolution should occur, it would theoretically be a long process.
It's not like you're a Transformer who can plug in a cable and instantly transform into an EVA, not even Gundam could pull that off, let alone MH.
As for the second reason... well, with the Umbrella Corporation in this world, it's definitely right to pin the blame on Umbrella when in doubt.
