Locke & Cutter & "play in the joints"

Consider this observation of Prof. Rick Garnett:

For starters, Cutter highlights the flip-side of the “play in the joints” argument on which the Court had relied in last year’s marquee religion-clause case, Locke v. Davey. In that case, the Court ruled (incorrectly, in my judgment) that the State of Washington’s decision to deny scholarship benefits to an otherwise eligible college student, merely because the student elected to major in theology, did not violate the Free Exercise Clause. The majority in that (non-unanimous) case recognized that the Establishment Clause would permit Washington to provide the scholarship, but insisted that the Constitution did not require equal treatment in this situation. There is, the Court stated, some “play in the joints” between what the Establishment Clause permits and what the Free Exercise Clause requires. In Cutter, the justices acknowledged that this argument cuts both ways: There is also “play in the joints” between what the Free Exercise Clause requires and what the Establishment Clause forbids.

What are your thoughts? Do you understand how the concept of "play in the joints" works? How it allows for legislation neither required by the Free Exercise Clause nor forbidden by the EC?