I don't understand what you are asking. As explained, the echo charge sits between two batteries
The EC sits between two circuits, and current passes through it.
For a clarifying thought experiment, imagine only a PSU (without current limiting itself) on the source side circuit, the only bank is a large LFP on the target side, none exists on the source side.
So the PSU being capable of delivering (A) 200A, as opposed to (B) 20A.
The LFP is trying to suck 1000A through the 15A straw. Which a lead Starter bat would not.
Is there any significant extra stress (reduced reliability / longevity) on whatever component(s) are limiting the current within the EC
under scenario A, as opposed to scenario B?
IOW, is the fact that the EC is "filtering down" to that maximum 15A rate more "effort"
when a very high rate is both demanded from the target side, and also
potentially available from the source side?
If your answer is "I don't know" that is of course just fine.