I am using openSUSE Tumbleweed on a reasonably fresh install.
I configured "mail server" through YaST2 and selected to use fetchmail as a
daemon.
On completion I checked the running services and I see the following:

ps -ef | grep fetch
fetchma+   980     1  0 22:15 ?        00:00:00 /usr/bin/fetchmail -d 900 -f
/etc/fetchmailrc

In other words the interval is set to 900.
However in the "/etc/sysconfig" YaST2 option, the interval is set to 600:

cat /etc/sysconfig/fetchmail | grep -v "#"
FETCHMAIL_POLLING_INTERVAL="600"

FETCHMAIL_FETCHALL="yes"

FETCHMAIL_SILENT="no"

FETCHMAIL_USER="fetchmail"

FETCHMAIL_EXPERT_OPTIONS=""

FETCHMAIL_RC_PATH="/etc/fetchmailrc"


Unfortunately I could not work out where the fetchmail command was being
constructed so I cannot help with a solution.

However, it could be possible to specify the interval and the daemon option in
the /etc/fetchmailrc file as the following entry:

set daemon 600