The difference is that with speed off the A/T might come forward due to Min Speed Reversion if they were not designed to retard. So the feature is not there "in case you forgot to retard", it is implemented because of what I've written in bold.
A/T come off 2s after touch down.
BTW, no single channel to touch down, right?
The only allowed possibility to touchdown with A/T armed, other than dual channel, is speed off, A/P off, right?