The mainstay of treatment for clinical DM in dogs and cats is insulin along with dietary modification. Goals include controlling BG below the renal threshold for as much of a 24 hr period as possible
That quote is on page 3 of the document. For most cats, renal threshold is somewhere in the high blue/low yellow range. Top of page 4 talks about using Lantus or Levemir. Maybe your vet is looking at an ancient version of the AAHA guidelines.