Spring certification has nothing to do with Java ceritification as such. None stops you to take Spring certification before Java certification. But if you have not done Java certification(OCP with JDK 8 or 11), I would suggest you go ahead with Java certification first and then jump into Spring cerification. The simple reason being since Spring is a framework built on top of Java. So it always makes sense to make the foundations reasonably sound and being certified before going to Spring. So please do 1z0-809 before.
The BS book only gives 4 functional interfaces to study for 1z0-815 : Predicate, Supplier, Consumer and Comparator. Does it mean other interfaces are not in 1z0-815? The actual syllabus does not mention any of them. So a little confused here