I wish I had your research skills. Where does it even say that?
The letters may have been chosen for early, limited 7-segment display drivers, such as a hex-to-7seg decoder. However, these fell out of use when transistors were no longer expensive in the 1990s and it made more sense to minimize the number of chips rather than each chip's complexity, so the main microcontroller used 14 GPIO pins or a single external LED driver to multiplex all 6 digits. Before the 1990s, it probably did not make sense to make this device digital at all, I think gyms would just use analog clocks with marked dials.
I asked ChatGPT and it said it stood for Custom (set your own timing program) and Free (workout as you wish and press a button to record the times for later use). Not a bad idea but I had told it F is for the active period and C is for the resting period, so it is obviously wrong. Another wording gave me Focus / EFfort and Cool-down / Catch your breath.
I tried the Ecosia AI chat too, pasting the post title and it replied with Focus and Catch your breath.