Nonclustered Index Rebuild Online on Heap Table
My question is: If you try to rebuild online just non-clustered indexes on a heap table with those also be single threaded or can those go parallel?

It can be multi threaded, the restriction is ONLY to alter table rebuild command when done with ONLINE option. I would like to quote from Enabling Compression on Heap MSDN Blog

(1) If you use ONLINE option then HEAP rebuild is single threaded. Note, this restriction is not there when you are rebuilding an index which supports multi-threaded ONLINE build.  However, in both cases, OFFLINE build does support multi-threading.

A non-clustered index rebuild can be multi threaded. Please note for index rebuild to use multiple threads the edition should be Enterprise edition or above( if there is such a edition). This is still true from SQL Server 2005 to SQL Server 2019.

For example, rebuilding a heap with three nonclustered indexes online:
ALTER TABLE dbo.Fish REBUILD WITH (ONLINE = ON);

The heap rebuild is serial:

...while the associated nonclustered index rebuilds are parallel: