It were my Intel HD Graphics driver settings to be blamed. When "Application Optimal Mode" is on and "Conservative Morphological Anti-Aliasing" is set to either Override or Enhance Application Settings:
-- then fonts are always AA-ed in SeaMonkey if HW acceleration is on, regardless of font-specific settings. Intel driver-specific settings may even cause visual glitches with fonts display:
This is a known Mozilla issue -- see bug #1145987.