To use the concatenated output as list input for a data validation:
Presuming you have concatenated list in column c starting from C2;
Step 1: Create a named range and enter this formula in the Refers to:
field;
=Sheet1!$C$2:INDEX(Sheet1!$C$2:$C$1000,COUNTA(Sheet1!$C$2:$C$1000))
Step 2: Use the name created as input in the data validation: