AFAIK, the fact that a text can be focused has nothing to do with the FocusControl property.
At least in Windows, text controls can be focused with tabulation keys (TAB or CTRL-TAB): using the WS_TABSTOP style for LTEXT/RTEXT/CTEXT controls. This is not bound to Delphi or Lazarus, but to Windows.
Delphi has just reproduced this behavior (so do Lazarus) with the TabStop property:
http://docwiki.embarcadero.com/Libraries/XE7/en/Vcl.StdCtrls.TStaticText.TabStopI admit it's a bit weird that nothing is visible when a text is focused, but anyway it's just working like this : texts get focus without any visible effect. BTW, there are not the only ones: Groupboxes, for instance, have the same behavior.
On another way, the FocusControl property is specific to Delphi/Lazarus-FreePascal (i.e. it's not related to Windows). It just set the focus to the desired control, if you press the mnemonic present in the caption of the concerned statictext/label.
So, to make it short:
- Tabstop: text can receive the focus with the tabulation keys (no visual indication),
- FocusControl: set the focus to another control, if text mnemonic (Alt+Key) is pressed.
** EDIT** That's also why Labels have a FocusControl property, but not TabStop/TabORder ones: they are not 'text controls' from the Windows point of view.