That's not exactly what the FDA says:
Accelerated Approval
When studying a new drug, it can take a long time - sometimes many years - to learn whether a drug actually provides real improvement for patients – such as living longer or feeling better. This real improvement is known as a “clinical outcome.” Mindful of the fact that obtaining data on clinical outcomes can take a long time, in 1992 FDA instituted the Accelerated Approval regulation, allowing earlier approval of drugs to treat serious diseases, and that fill an unmet medical need based on a surrogate endpoint.
Your translation of "unmet medical need" into "no approved therapy" is sloppy, at best, especially in the case of T-DMI, where women with metastatic breast cancer had failed, on average, seven previous treatment regimens for the disease. A drug for women failing seven previous treatment regimens for their disease would seem to most reasonable people to fill "an unmet medical need."
Roche had a pre-submission meeting with the FDA. I can't imagine this issue didn't come up in the meetings.
Bladerunner