I think FTE is mostly used as a 'unit'. E.g. if two people work on something 50% of the time, you get one "FTE-equivalent", as there is roughly one full-time employee of effort put in.
Though in this context it just seems to be the number of people working on the code on a consistent basis.
* “Full Time Employee” (which can itself mean “not a part-timer” in a place that employs both, or “not a temp/contractor” [in which case the “full-time” really means “regular/permanent”]) or
* “Full Time Equivalent” (a budgeting unit equal to either a full time worker or a combination of part time workers with the same aggregate [usually weekly] hours as constitute the standard for full-time in the system being used.)
Though in this context it just seems to be the number of people working on the code on a consistent basis.