You're absolutely right, the TCA9548A and HT16K33 share the same default i²c address of 0x70! They also share the same alternative addresses of 0x71 to 0x77.
It won't be a problem. You only need one address for the TCA9548A, so choose one and avoid using that address for any of the HT16K33. For example, connect A0,A1,A2 to Vcc and the TCA9548A will have address 0x77. Then give your HT16K33 addresses in the range 0x70 to 0x76, but make sure they are unique on any single output channel of the TCA9548A.
For example you could set the addresses of 4x HT16K33 to 0x70, 4x HT16K33 to 0x71, 4x HT16K33 to 0x72 and 3x HT16K33 to 0x73. Then connect 4, one with each address, to TCA9548A channel 0, 4 to channel 1, 4 to channel 2 and the final 3 to channel 3.