A synchronous transmission is a transmission, where the transmitter sends both - clock and data, and the receiver doesn't have an own clock. When receiver and transmitter both have there own clock, these cannot be synchronous - they always will differ a little bit, what makes them asynchronous.
But there a two very different techniques of transmitting the clock. This can be done with an own clock line ( as with SPI or I2C). But it is also possible to transmit clock and data on the same transmission line (as with an USART Interface). The receiver has to retrieve the clock from the datastream.
Maybe that's what @Paul_KD7HB has in mind.