They can use the peg out mechanism which is no secure way to get your coins the bip300 is more like a trust me bro transaction.

When two large mining pools vote for a peg out no one can block it if they have 51% of the hash rate.

When less than 50% of the miners support a specific chain a peg out is not even possible.

Also bip300 leads to mining centralization. Because you have to validate the sidechains. That’s not possible for small miners.

As I already said, it’s complete bullshit!