The difference is in their plan to lease the sets, and the difference in how that's accounted for. Setting aside the question of "where will they find something to lease" It's ultimately a time value problem - even if it costs more in the long run. Financially, leasing means they limit upfront expenditures and only need to commit 5m dollars in each of the next 10 fiscal years, for example. (Not including whatever work they'll need to do regardless to be able to use BEMUs).
Building out the system and buying traditional EMUs means they have to find the room to program ~100m into the CIP as a lump sum. In FY25, the CIP only had ~850m in funds available for programming, most of which went to Draw 1.
Also, as basically a long term rental, it can represent de-risking - if BEMUs fail to work out, or the technology improves, the T can return them to the lessor at the end of the term, presuming of course that replacements are in place. (That is the pitfall that Amtrak is falling into with the lessor owned Acela Is).