Did you read the article? Here's an extract of the key part:
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Ultimately, this is not ideal, and while it would no doubt create confusion, it would not create a complete hard fork and should not lead to anybody losing Bitcoin.
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This doesn't make sense to me.
What it is saying is that, to get around a hard fork (and a soft fork), some miners will mine a new block type.
This new block is at 1MB, but can "expand" to bigger than 1MB, by allowing included txs to fall outside of that 1MB limit
and into the "mushroom" section of this new block. When you are in this section, you can not see, verify, or spend (technically).
That means when people use bitcoin, they won't know if their tx will be within the 1MB or the mushroom section.
This alone will cause too much damage to the market place, especially with merchants.
Bitcoin is based on a provable tx. Your now saying we won't be able to prove or see some txs.