Joking, whether it is based on truth or lies, has many scenarios: 1- when the joke is based on truth, and there is no lying involved; this is permissible; 2- when the joke is based on a lie; this is prohibited; 3- joking by telling stories, when you are not certain whether they are true, but they could have happened, what appears to be the case, based on the actions of some scholars is that this is permissible. Interesting and funny stories are of three types: 1- stories which we know to be true; there is nothing wrong with this type; 2- stories which are known to be lies and did not happen, it is impermissible to tell these stories; 3- stories regarding which it is not known whether they happened or not; there is nothing wrong with telling these stories.

You are right to tell them off, because they are coming close to kufr with these words. Changing one letter does not alleviate the grievousness of their error.