Well, they are both progression currencies. You can't buy ships or modules for mats, though, and the only things you really need in the game are ships and modules. So credits represent the core progress counter. Yet we can get done with the progress they represent in a very short time.
Exploration rank has also been rendered near worthless, as it used to be a counter for how far you had been away from the core systems. Now you can bring the UC with you on a carrier, meaning you can obtain the payout, tags and rank without actually going back to civilisation where the data is supposed to have actual value!
FD seems hellbent on removing any progress and challenge from the game.
S
I would bet fdev rolls out another new currency with odyssey - so we will have credits as currency for base stuff, mats as currency for improving stuff, and whatever X odyssey collectible to get / improve planetary legs gear