True RMS earth leakage relay
The relay ... claims to trip in less than 25 ms to fault currents that are 5 x greater than the set trip current.

That should be OK. For a sinewave the peak current would be \$\sqrt 2 V_{RMS}\$ so five times would indicate a pretty severe overload.

Also the minimum time settings are inst (what time is &quot;inst&quot;) and 50 ms.

That's the Δt described in the timing diagram. The INST setting will initiate the tripping immediately the current exceeds the threshold. There will be a reaction time for the relay.

How can one find reliable RMS for both 50/60 Hz in that short amount of time?

It don't think it claims to use RMS readings for the instantaneous settings. I note that the next setting is 50 ms so that would be 2.5 cycles on 50 Hz, the lowest mains frequency it's rated for.
The minimum time interval where both 50 and 60 Hz signals are almost synchronized is 100ms (6 cycles of 60Hz and 5 cycles of 50 Hz).