Behaviour of parallel degree 1 vs. noparallel on index.

We have a large 19.13 DB running on Linux. In the entire system, only 2 indexes had a degree of parallelism on them and I was asked to change them to parallel degree 1. No other objects have parallelism defined. Normally I'd just alter to noparallel but looking on the internet there appeared to be no difference in altering to noparallel or parallel degree 1, both should cause serial execution. And both look the same in dba_indexes.

Very complex SQL uses the index. There are no hints in the SQL. After modifying index to parallel degree 1, several portions of the SQL invoke parallel execution to the extent the statement uses 64 parallel threads. When I modify the index to noparallel, it runs serially as expected.