It seems that your own article contradicts its self, not to mention ANY cites and the vast number of genetic twin studies for autism, asthma, et al…that claim concordance of 100% is THE standard.
“However, with almost no exceptions, the DNA of monozygotic twins will be identical, making a genetic search much easier…However, with almost no exceptions, the DNA of monozygotic twins will be identical, making a genetic search much easier.”
In fact the proof of MZ is quite hard to obtain, how many placentas did the Brazilian twins have? How was Zygosity proven and that it was not commonly DZ? This article is an anomaly and probably commercially driven.
Autism: "Concordance rates among identical twins are estimated to be approximately 60%, but have been reported to be as high as 95%."
Asthma: "The table shows, however, that instead of a 100% concordance, there is only a 59% concordance between identical twins."
As far as how it's "proven". DZ twins share only 50% of their DNA, same as any pair of siblings. A single mutated gene out of how many genes in humans? Close enough that we're not going to be seeing a third twin category -- they are "identical".