Bitcoin will just softfork - remove old signature scheme

[MartinS84]

Here: https://faqq.info/but-bitcoin-will-softfork/
it says: "To have an actual quantum resistant blockchain, you need a hard fork, and remove the old signature scheme."

Thats not correct.
A softfork is a restriction of the rules.
removing old signature schemes is such a restriction of the rules.

Old nodes will accept all blocks by new nodes.
But not the other way around.
A block not containing any transaction signed with the old signature scheme, is still valid according to the old rules. In fact we already have valid blocks not containing any signatures at all, empty blocks.

[MartinS84]

"In a soft fork, the old rules are still allowed to apply and an extra rule would be added."

Its kind of the other way around.
A softfork adds new rules, but within the constraints of the old rules.
An analogy is vegetarianism vs veganism.