why 'increasing the block size above 1 MB requires a hard fork.” actually not require fork? in this link, they say 


  “increasing the block size above 1 MB requires a hard fork.” In this
  example, an actual block chain fork is not required—but it is a
  possible outcome.


but i can't understand why actually it not require block chain fork. when block upgrade, soft fork or hard fork will occurred. but it say both of fork is not required actually.

thanks for your help :)
The definitions here are a bit confusing. It is important to understand the distinction between the "soft/hard fork" and the "blockchain fork".


Blockchain fork is an actual split in a chain of blocks. If two groups of people run incompatible software - two different chains will emerge. For example, Bitcoin and Bitcoin Cash follow two different chains that used to be one chain.
Soft or hard fork is a mechanism of performing consensus rule changes. Hard fork upgrade has a risk of causing a blockchain fork, but it does not have to cause it. If everyone will upgrade in time, there would be no chain split. For example, Ethereum has performed multiple "upgrades" through hard forks (such as Byzantium), and the chain did not split. Bitcoin prefers to stay on the safe side and introduces most of the changes to the protocol as soft forks.


To add to the confusion there are also user-activated forks and miner-activated fork, but these are not relevant to the question.