When a workbook that is not checked out is opened in Excel 2016 using DMF it is opened in read-only mode regardless of the Disable auto check-out on open setting.
This is because the default behaviour when opening Excel files from SharePoint using Excel 2016 has been changed by Microsoft (see below blog for more information):
Microsoft’s rationale for this change appears to be to ensure that Excel documents are not opened from SharePoint in edit mode without the user consciously selecting ‘Edit Workbook’ in order prevent locking out other users from editing the file. MacroView's philosophy with regard to DMF functionality is to enhance the experience when working with SharePoint while respecting standard Microsoft functionality and as such does not change this default behaviour.
The impact of this is that Excel documents opened in Excel 2016 via MacroView DMF will always open in read-only mode unless both auto checkout is enabled (i.e. Disable auto check-out on open setting is unchecked) and OpenDocsReadOnlyByDefault is disabled, or if Disable auto check-out on open is checked and the document has already been checked out to the current user.
If you want Excel documents opened via MacroView DMF to be checked out by default we recommend the Disable auto check-out on open setting is unchecked and you make use of the right-click Check Out option before opening Excel documents for editing (as is currently required for all document types in the browser).
If you do not want Excel documents opened via MacroView DMF to be checked out by default then check Disable out check-out on open under DMF Options, then after opening the document, select Edit Workbook if editing is required.
Note: When using MacroView AOI/UDN, Disable auto check-out on open must be unchecked.