Okay - ignore the critic markup bit from earlier. With a bit more research I’m not convinced that is true … probably just matches where I’ve seen it.
I haven’t been able to force the MMD engine into recognising it except by being heavy handed. Until such time as a solution is published here, the following heavy handed aproach of substituting the tilde pairs for ‘del’ tags should offer an interim solution. You can then modify the intial script based substitution action to include any additional markups that may not be available, but have HTML tags defined or CSS transformations … though you might also want to amend the preview action step as well, in order to support that.