I suggest that the premise of this question is somewhat incorrect. The umlaut dots cannot be written as a single line, but a single line is well what might end up on paper when writing quickly.
The faster you write, the higher becomes the chance you might just not sufficiently lift up the pen from the paper while moving between two points that are supposed to be disconnected in theory. That is why in most types of "Schreibschrift", more or less all letters are joined and reshaped in such a way that large parts of words can be written without lifting the pen up from the paper.
In order to write an umlaut, the writer has to place two dots right next to each other in short succession. Some writers might not lift up the pen sufficiently to avoid leaving a line between the points, and some may have stopped bothering altogether and just move from the left dot to the right dot without even attempting to lift the pen. (Note that we are talking about movements that take less than 10ths of seconds.)
To respond to your question: The umlaut dots cannot be substituted with the macron diacritic, but as German does not use a macron (and the few words that use an acute accent can be recognized from context), there is no danger for dots that look like a straight line to be misread as anything else. Or, as the difference here is quite subtle: If you ask a person whose handwriting features umlaut dots that look like a horizontal line to write very slowly, those dots should morph back to separate dots.
By the way, the same applies for a squiggly line: In handwriting, the umlaut dots are often written like a double acute accent instead of dots (again, because getting those two little right is easier for some people than getting two dots right when writing fast). If the writer does not properly lift the pen between the two "acute accents", the result is obviously a squiggly line.