Its not easy to explain. Zhiyi expounded a lengthy teaching on the name of the Lotus Sutra - Profound Meaning of the Lotus Sutra. In that text, Zhiyi explained that the title invoking the metaphor of the Lotus is to help people understand the teaching. But, the distinction between symbol and symbolized is intimately related when we consider through the perspective of the Middle in the Three Truths teaching, so that the name and the thing carrying that name are continuous (two but not two).
He explains that there are three Lotus metaphors that explain the Trace teachings and three metaphors that explain the original teaching. Unfortunately, we don't have a complete English translation of the Profound Meaning of the Lotus Sutra. We have a partial translation found in Swanson's T'ie T'ai Philosophy. This translation does not include the section on the metaphors. We also have a section by section summary of the text by Haiyan Shen. There are translations of passages of the text scattered throughout academic articles and books. There are also passages translated as part of translations of Nichiren's writings because he quoted these texts in his own writings. Here's a passage contained in one of Nichiren's writings:
Question: Does the term (Lotus) renge in fact mean the lotus, that is the essence of the meditation based on the Lotus Sutra? Or does it in fact mean the ordinary lotus that is a species of plant?
Answer: It in fact refers to the lotus that is the essence of the Lotus Sutra. But because the essence of the Lotus Sutra is difficult to understand, the metaphor of the lotus plant is introduced. People of sharp faculties will hear the name and immediately grasp the principle. They have no need to rely upon a metaphor but can understand the Lotus Sutra directly. But people of intermediate or inferior perception will not understand immediately. Only through the medium of a metaphor will they be able to understand. Thus the easily understood metaphor of an actual lotus plant is used to make clear the difficult-to-understand lotus that is the essence of the Lotus Sutra.
Thus, in the Lotus Sutra the Buddha employed three cycles of preaching in accordance with the respective understanding of those of superior, intermediate, or inferior capacity. For people of superior capacity, the renge, or lotus, that is the name of the [Dharma] was taught. But, for people of intermediate or inferior capacity, the lotus was used as a metaphor or symbol. As long as one understands that the word is being used both as a name for the [Dharma] itself and as a metaphor, depending upon which of the three groups of people is being addressed, then there should be no reason to argue over it.
To say the title is the Tathagatagarbha... I suppose you can get to that interpretation since the Buddha is the Dharma, and the Dharma is the Buddha (Buddha is said to have remarked, "Whoever sees the Dhamma sees me; whoever sees me sees the Dhamma.") AFAIK, there is no prominent Tendai teaching that equates the title of the sutra with the Tathagatagarbha. The title does not carry the significance that Nichiren read into it.