It may or may not be illegal, they're just throwing their weight around under the suggestion that it's illegal. Knowing well that a single dev working on a plugin in their freetime isn't likely to want to invest in legal proceedings.
Which is even more reason to do the above, to stick it to them.
Why would it be illegal? A cease and desist is just a fancy letter saying do what we want or else we'll go crying to a judge. If the original dev hasn't done anything wrong (I'm not familiar with the project but I'd still say that's highly likely to be the case) then there's nothing to worry about. So make all the clones you want, and watch a dumbass company Streisand itself
It wouldn't be illegal if you hosted it in a country where there is no law against such a thing, and/or Haier does not have any presence which would grant them legal standing. Is anyone running a server farm in Sealand yet?