You are confusing the widget path component, an arbitrary string of chars other than the separator '.', with the English word 'frame', which is also a tk command.  In tk docs, 'a *frame*' is a widget (tk window) created with the 'frame' command.

By default, tk gives each widget(window) a unique but meaningless string of digits as its path component.  A user can override this by giving an explicit name option on creation.  

A few years ago, tkinter started always overriding the tk default with a  readable name.  Serhiy and I agreed on using a '!' prefix since it was unlikely though not impossible to be used by tkinter users.  We must have agreed on not specifically flagging ttk widgets in the path component, as opposed to the widget name (see below).  A numerical suffix is added to duplicates.  The system is not perfect, but overall we prefer it to the tk digit sequences, in spite of the following possible confusion.

>>> c = ttk.Combobox(r, name='!frame')
>>> c
<tkinter.ttk.Combobox object .!frame>
>>> c.widgetName
'ttk::combobox'
>>> c.master.wm_manage(c)
...
_tkinter.TclError: window ".!frame" is not manageable: must be a frame, labelframe or toplevel

For debugging in 'python -i' or IDLE 'run module' mode, one can followup by checking the passed-in widget's widgetName if it is not otherwise obvious.

If you want tk or its tk wm_manage doc at https://www.tcl.tk/man/tcl8.6/TkCmd/wm.htm#M53 changed, you have to ask the tcl.tk people.  Extremely unlikely I suspect.  I presume that there are technical reasons why a ttk Frame is not a drop-in replacement for a tk Frame in this context.  And since a *ttk::frame* is clearly different from a *frame*, the doc is clear enough.  For all other commands, the window can only be a *toplevel*, so the exception for wm manage is an extension to frame and labelframe, rather than a restriction.

Our 3-paragraph doc for the corresponding Wm class does not discuss the individual methods but implicitly defers to the tkinter.__init__ code and the tk doc.

Our ttk doc does not discuss the ttk widgets that are essentially copies of tk widgets except the mostly documented differences.  So there is not  a good place to say anything and I am dubious that we should.

So I think we should close as 'not a bug' unless Serhiy disagrees.