Short explanation why:
Smartmeters have 2 IR outputs: One issues SML records every second specified in BSI TR 03109 1.
The other one is a test IR output issuing - a two ms pulse at a rate 1 pulse per 0.1 Wh.
So consumption equals the inverse of the distance between two rising pulse edges.
Finally found it, which I did not expect, usually I am doing MAXes or STMs: That thing is so extremely small and stripped down, it just does not have any inbound interrupt processing.
E.g. mentioned here PRU interrupt program
Verified with the instruction set documents: No such thing as an RTI.
But I can still implement it as a polling task with possibly a bit of debouncing.