How does less cable impedance, source/termination impedance, lead to longer cable length for higher data rates?
I understand you asking these sort of questions and I've tried to answer in the format that you presented the questions. Please forgive if I have misconstrued something.

Q1 - cable length when transmitting high data rate: -

(1) Impedance of the transmission line is independent of cable length

(2) Source impedance is independent of cable length

(3) Termination impedance is independent of cable length

Q2 - data rate for a fixed length cable: -

(1) Impedance of the transmission line is generally higher for better data success

(2) Source impedance can be zero for improved results providing the receiver has good terminator

(3) Termination impedance needs to be the correct value for the cable to minimize reflections corrupting the data