See related discussion between @schien and @tomoyukilabs on Web Platform Tests repository: web-platform-tests/wpt#3844

In particular, @schien notes that:

However, in the second note it says "setting presentation.defaultRequest" should take no effect. I take this sentence as "assigning value to defaultRequest will be no-op", which means presentation.defaultRequest will still be null after the assignment. That's my interpretation.

To which @tomoyukilabs replies:

I'm afraid that the spec might look ambiguous. I regard it as "Although the defaultRequest property can be set to any instance of PresentationRequest or null, browsers which do not support defaultRequest will never refer to its value unless a script accesses it directly".

I would read the spec as @tomoyukilabs. Regardless of who is correct, it seems a good idea to clarify the expected behavior!