The reason the resemblance is ironic considering these two escape plans is that the elaborate nature of Huck's escape was necessary and the elaborate nature of Jim's escape was not.
Huck had to plan carefully so that he could 1) escape from his father and 2) convey the idea that he has been killed. Huck does not only intend to escape, but to fool his father (and the town) into thinking he is dead. For this reason, Huck has to concoct an elaborate plan, complete with detailed "indications" of murder.
Jim, conversely, only needs to escape. The elaborate plans that Tom makes to help Jim escape do not actually help but only add "adventure" to Jim's liberation.
The irony rests in this discrepancy. We read Huck's escape plan as something of necessary genius and the plan for Jim's escape as genius applied groundlessly.