I've never seen anything except this provide passive forms of the verb esse. And even with that most of the passive forms are crossed out. Why is this? It would make sense for there to be no passive forms of esse, because of the structure of sentences with est: nominative est nominative. There is no object to become the subject, but then why does the website provide passive forms? How would these work? Or are these just approximations to what such a word would look like?
There are no passive forms of esse, for the reason you state -- it's not a transitive verb. Intransitive verbs cannot be passivized, with the minor exception of "impersonal passive" forms (in the 3sg. only), which are not used with esse. The crossed-out forms on that site don't exist. (Neither do some of the forms that aren't crossed out, like the gerundive esendus and several others. On the other hand, the "II imperative" forms actually do exist even though they are crossed out for some reason.) Presumably the site generates forms by an algorithm, which doesn't necessarily know which forms really exist.
This thing you linked to is total rubbish. It was obviously produced by a machine. The passive forms given there are fictitious. The verb "to be" does not take a direct object and thus cannot be passive; this is true in Latin and all languages. I suggest you buy a Latin grammar and stop using machine-generated websites.