Is that true? Antoine claims [1] that opt-in RBF isn't enough to avoid
a DoS issue when utxos are jointly funded by untrusting partners, and,
aiui, that's the main motivation for addressing this now.

[1] https://lists.linuxfoundation.org/pipermail/lightning-dev/2021-May/003033.html

The scenario he describes is: A, B, C create a tx:

  inputs: A1, B1, C1 [opts in to RBF]
  fees: normal
  outputs:
    [lightning channel, DLC, etc, who knows]

they all analyse the tx, and agree it looks great; however just before
publishing it, A spams the network with an alternative tx, double
spending her input:

  inputs: A1 [does not opt in to RBF]
  fees: low
  outputs: A

If A gets the timing right, that's bad for B and C because they've
populated their mempool with the 1st transaction, while everyone else
sees the 2nd one instead; and neither tx will replace the other. B and
C can't know that they should just cancel their transaction, eg:

  inputs: B1, C1 [opts in to RBF]
  fees: 50% above normal
  outputs:
    [smaller channel, refund, whatever]

and might instead waste time trying to fee bump the tx to get it mined,
or similar.

What should folks wanting to do coinjoins/dualfunding/dlcs/etc do to
solve that problem if they have only opt-in RBF available?