SASI SPARSE mode 5 limit

I do not know what the "improvement" should be here, but I ran into this:

https://github.com/apache/cassandra/blob/cassandra-3.11/src/java/org/apache/cassandra/index/sasi/disk/OnDiskIndexBuilder.java#L585

Term '55.3' belongs to more than 5 keys in sparse mode, which is not allowed.

The only reference I can find to the limit is here:
http://www.doanduyhai.com/blog/?p=2058

Why is it 5? Could it be a variable? Could it be an option when creating the table? Why or why not?

This seems awkward. A user can insert more then 5 rows into a table, and it "works". IE you can write and you can query that table getting more than 5 results, but the index will not flush to disk. It throws an IOException.

Maybe I am misunderstanding, but this seems impossible to support, if users inserts the same value 5 times, the entire index will not flush to disk?