By "legacy" they probably mean that they work with the older process technologies, not that the fab itself is old:

The acquisition includes an existing 300 mm fab cleanroom of 300,000 square feet and will further position Micron to address growing global demand for memory solutions

In its May 2024 ’Hooray, we’re open!’ announcement, PSMC said it invested more than NT$300 billion (US$9.5 billion) on the facility, and that it had capacity to produce 50,000 12-inch wafers per month under 55, 40 and 28 nanometer technology nodes.

Those kinds of chips are still very useful for things like cars and washing machines and such where you don't need bleeding edge chip tech.