I just looked at the article you mentioned.

It states:

At the EIA-232 serial port, voltages are bipolar (positive or negative with respect to ground) and should be about 12 volts in magnitude (some are 5 or 10 volts). For the transmit and receive pins +12 volts is a 0-bit (sometimes called "space") and -12 volts is a 1-bit (sometimes called "mark").

The author means to say that a 0-bit is known as "space", no matter what voltage level is representing the 0-bit.