It's not merely sensing heat, but sensing weakly radiating heat from a distance. The human researchers could only distinguish the test and control objects by touching them.
All the article says is that the nose of dogs is much more sensitive to heat than other animals, calling it a "new sense" is an exaggeration.
Humans also sense radiative heat from a distance. The only difference explicitly stated in the article is degree of sensitivity not a new sense.
But it can be a nuisance.
I think degree of sensitivity, discrimination, or directionality can warrant counting a 'new sense'. I would consider my sense of thermoreception to be different from that of a snake's heat-pits, or hearing to be a different sense from feeling vibrations.
As the article about epistemology posted yesterday mentioned, at some point, a difference in degree becomes a difference in kind.