It's strange but the > and < operators work in my version, QV11.2 SR3.
I suppose another way might be to reference variables in your dynamic command to only use the = operator, maybe something like ...
UPDATE MovimentiFinanziari SET PrendiNonPrendi = 'S' WHERE SubStringCount(vVar,DataMovimentoTXT)=1;
... and vVar is a distinct, and ordered ascendingly, concatenated list of values split at the relevant point ...
... and vGT is a variable holding the 'greater than' value.