What the sound engineers told me is correct. Although there is to date no easy cure, the invertion is the base to re-work the reverb "removal".
It's doubling the stereo track, inverting the doubled one so the sound is completly cancelled. Then re-work the frequencies (re-create) from both so you obtain a very flat mix, leaving cancelled the frequencies where the echo is most present. (no possible real cure with today's tech for serious reverb)
then do a mixdown and from the mixdown, recuperate the frecuencies that have been canceled previously. As the reverb is "sort of" removed, the rebuilding equalization of a decent sound is possible.
It's not an easy turnarround, to a specialist level, but this is the path and with a bit of patience a lot can be fixed if the reverb is not too harsh.