DB2: how to export data file and own by db2inst1 via ADMIN_CMD
This is by design; ADMIN_CMD and other fenced routines execute using the fenced user authority in order to protect ("fence") the instance from buggy or malicious code. 

Some people set the fenced user to the instance owner ID during the instance creation, which would cause fenced routines to run with the instance owner authority. However, this creates a serious security vulnerability. 

It's better to ask your system administrator to allow the instance owner limited sudo or runuser rights to execute the necessary commands as db2fenc1.