I think that is kind of a half-truth, all types of Ethernet are synchronous in the sense of not having start and stop bits. The only important difference between 100BASE-T4 and the rest is that it uses all four pairs of cat3 at a lower modulation rate than 100BASE-TX (which requires cat5 because of its higher bandwidth). The data signal is differentially driven over twisted pair, and is self-clocking. On lower speed links that is achieved with Manchester coding, which is a self-clocking code with a coding efficiency of 1:2, whereas on higher speed links like 1000BASE-T it is a code like 8b10 or 64b66 with much higher efficiency.
The clock is extracted by having the sender and receiver both use an accurate reference frequency, sending a framing burst before each data packet, and a special PLL in the receiver that locks onto the frequency of the framing burst. Similar techniques are used everywhere, from CD-ROMs to wifi.