That change is rule breaking thus it is a hard fork, just sold as a soft fork
Soft forks add rules. Hard forks remove them. No consensus rules are being broken by Segwit.
Don't make sure conclusions when you only see 10% of the whole picture. Check core's definition of soft fork, it is not that simple, adding rules is only one out of many possible scenarios of soft fork
https://bitcoin.org/en/bitcoin-core/2016-01-07-statementI can change the transaction format in my node software, and that change will split the chain immediately, since the old nodes do not accept my transaction format and my node do not accept old transaction format. But that's only one out of 4 different scenario: I can reject old transaction format while make my transaction format acceptable to old nodes, I can accept old transaction format while make my transaction format unacceptable for old nodes, or I can make them acceptable both way
That gives 4 different scenarios, and depends on if the hash rate of new nodes are larger than old nodes, in each case there will be 2 different scenarios (except the last) : either one of the chain get orphaned, or both chain extends indefinitely. That's total 7 different scenarios, while in 3 of them the network will partition into two different chains
And this is only for normal soft fork scenario, not segwit like soft fork. segwit soft fork is a special type of soft fork that does not follow the above rules since it created a virtual bitcoin node which looks exactly like bitcoin node and talk to bitcoin nodes, and the old network thought that they are talking to a real bitcoin node while in reality they are not. So a segwit node can break any rules while old nodes still believes they are talking to a normal old node. This action in itself is more like an attack to the network instead of an upgrading, thus is criticized by knowledgeable people