From the Activation section, which I should have included in the OP, but hadn't read it yet.

Reactive DeploymentReactive Deployment

Doesn't this proposal activate too soon?

In short, yes, it does. Unfortunately, due to the release of Bitcoin Core 30 endorsing large data storage and actual illegal content being mined in proposed block , the network as it stands is already contaminated. This is not a preemptive measure, but an emergency response to an immediate crisis. Therefore, there is no time for lengthy signaling periods or careful deployment; the only remaining option is immediate and retroactive activation to mitigate the harm.

However, because this is a softfork, old nodes will continue to follow the blockchain so long as it attains significant enough hashrate. Even if non-upgraded miners continue to mine (now) invalid blocks, old nodes will continually replace them with valid blocks every time the valid chain overtakes the invalid one. Since this results in an incoherent consensus system, unusable for actual currency, and loss of rewards for the lagging miners, they should quickly adapt and bring the softfork to near 100% compatibility quickly.

The true risk is that for an initial period of time, nearly all nodes will cease to function as fully validating nodes. This can only be mitigated by encouraging nodes to upgrade quickly, possibly by backporting the softfork to old and/or niche node software as necessary. Toward this end, the actual changes in this softfork have been kept very simple.