Yes, the paper makes clear that the risk of dogs contracting the virus is a major problem to solve - especially since a symptom may be loss of smell! No reports yet of dogs spreading covid though.
AFAIK most dogs can't get Corona (for now). Dogs that have tested positive for corona are rare. There is no report from Dog to human infection.
Rare, but this method is asking them to smell saliva samples all day. That’s a level of exposure very few dogs would have.