By not providing any material to support the asterisk, the firm placing the ad lost an intended opportunity to clarify what they meant by the word "free" and are now left in the same position as if the asterisk had never appeared at all.
Realistically, somebody connected with the firm publishing the advertisement probably planned to clarify the meaning of the word "free" to give it a more narrow meaning than its undefined meaning without any qualifications. But apparently, somebody failed to get around to doing that or it was lost in a typesetting error. So, any restriction on the meaning of the word "free" that was intended by the firm has been lost, potentially causing the offer to be abused in ways that the firm hadn't bargained to commit itself to.
It pays to proofread anything that will be distributed to the general public because it can weaken the publisher legally by not saying something it should or by saying something inaccurate, and also because it makes the publishing firm look incompetent.
UPDATE FROM COMMENTS:
By the way, the flyer came with a gift card sticked onto it. The
matching asterisk is explained on the back of the gift card, which was
not visible while it was sticked onto the flyer. The matching asterisk
reads "with an order value of at least 10.-, only redeemable online".
Do you think this is still acceptable?
This is probably still effective to limit the offer. While it isn't readily visible, it is visible before you actually try to use the offer.