Sorry for the nitpick but こんにちは. Writing the greeting with わ is undoubtedly an informal version, but we prefer the version you can use wherever you go. After all, this is a language learning site, and you need to tolerate some educational high-handedness.
Now, the difference of the verb forms. It's not about transitive v. intransitive. It's more to do with aspect. Aspect (as opposed to tense and mood) is an expression of what part of the action denoted the speaker wants to focus. Is the speaker talking about the whole action? or or only a part of it?
The teiru-form is the marker of the imperfective aspect. By imperfective I mean part of an action, or an action that is so big that it continues into the time outside the framework of the speaker's speech. The markers of perfective aspect are -ta and -ru. Whether the action is located in the past, the present, or the future, perfective aspect means that the speaker wants to talk about it as a single unit, small enough in size to be fitted into the speaker's frame of reference.
So much for the theoretical basis. The imperfective いっていました means that Tanaka was still "saying" that he would be off from work tomorrow when the speaker finished their speech. In other words, there will be a surprise if Tanaka shows up to work next day. In not-so-much stark contrast, the perfective いいました refers to a completed action. Typically, the utterance is followed by a description of what immediately followed Tanaka's remark. It may be a colleague gossiping about Tanaka's health or the boss showing displeasure, and so on. I don't necessarily suggest that the boss overruled Tanaka's leave, but it will come with far less surprise if Tanaka could not take a day off than it would be with the perfective いっていました.
