From MSDN's note
https://learn.microsoft.com/en-us/sql/t-sql/database-console-commands/dbcc-shrinkfile-transact-sql
You can reduce the default size of an empty file by using DBCC SHRINKFILE target_size. For example, if you create a 5-MB file and then shrink the file to 3 MB while the file is still empty, the default file size is set to 3 MB. This applies only to empty files that have never contained data.
I have never seen term called 'Default File Size' except of the file size of Model Database, Are they referencing initial size of the data file, or something else that I am not aware of?
YES,the default size here means size of file when it was created..It can be see further down in docs
The default size is the size specified when the file was created.