Again, FERPA does not prevent any teacher, US or abroad, from showing the rec to the student. What FERPA does do is that it protests student records and applies to any colleges that receives federal funds. Once that rec is sent, it becomes part is the student’s records. The law says nothing about what the teacher does with her copy.
By waiving FERPA , the student is waiving the right to see that part of his/her records. Many feel that a rec without the waiver will be less forthcoming and, at least ib the UD, many teachers will not do a rec unless FERPA is waived.
But getting back to the OP, not waiving will probably not impact admissions, unless the recs were what would have moved the needle from undecided to admit.