Applying zero-phase filtering in the frequency domain - also works in real-time?
It's not a loophole.  A causal linear phase filter is identical to a non-causal zero phase filter in series with a delay of half the filter length.  If you check the results of your filter with an impulse input, you will probably see such a delay.
So it's not really real-time if you count the delay, but is often close enough to seem real-time if the filter's impulse response and/or block size is short relative to your real-time requirements.