The author, like most people, is confused about the issue. He says we risk splitting into two with the hard fork. This is utter nonsense, the fork will not happen without 90% consensus. The remaining 10% will then be forced to join or be worthless.
The author's analysis is reasonable. I don't know where you get "90%".
Keep in mind that there are two "forks" here:
One is the fork in the software (implementing a max size of 1 MB or 20 MB). A hard fork is one in which the two versions are not interoperable. This change is a hard fork because the 20 MB version accepts blocks that the current protocol rejects.
The other is a fork in the block chain. The block chain is really a block tree where only one branch persists for any significant amount of time.
The result of a hard fork in which both protocols persist is a persistent fork in the block chain. It doesn't matter how many people use one side or the other. It can be 50/50, 90/10, or 99.99/0.01. As long as both sides are used, the fork is persistent.
Normally, one side of a hard fork quickly gains adoptance and the other side must conform or become obsolete. The author is claiming that if a significant percentages continue on both sides then the confusion and problems will break the faith in the system, and people will abandon Bitcoin.