The announcement of the original T5Gemma goes in some more detail [1]. I'd describe it as two LLMs stacked on top of each other: the first understands the input, the second generates the output. "Encoder-decoder models often excel at summarization, translation, QA, and more due to their high inference efficiency, design flexibility, and richer encoder representation for understanding input"

1: https://developers.googleblog.com/en/t5gemma/