Markers can be added only on clip selected by timeline ruler.
Here a video about this:
this probably was made because the ruler is much more precise than mouse... maybe it's necessary to obscure the marker creation choice when the ruler is not present. or to have different voices for different clips (for example "Add marker" and "Add marker from pointer" )
P.S. the add marker/guide quickly is creating always the guide on the clip with the timeline ruler (also if this clip is outside view when the command is selected from another clip)