Thermal radiation is inherently broadband and incoherent, limiting its utility in applications requiring spectral selectivity such as molecular spectroscopy and free-space optical communications. Bound states in the continuum provide a powerful route to confine light in open photonic systems and realize resonances with extremely high quality factors. Here we demonstrate high-Q light emission from a photonic-phononic nonlocal metasurface composed of monocrystalline silicon on a 4H-SiC substrate. The platform supports BIC-enabled ultra-narrowband emission for both TE and TM polarizations; specifically, we identify an off-Γ Friedrich-Wintgen BIC and a Γ-point symmetry-protected BIC for TE polarization, and a Γ-point symmetry-protected BIC for TM polarization. Under TM polarization, coupling between the photonic mode of the metasurface and surface phonon polaritons in the SiC substrate produces strong field localization and a symmetry-enforced complementarity between the photonic and phononic resonances, in which the photonic Q-factor diverges while the phononic mode radiates maximally at the Γ point. Using a bonding-based fabrication approach that integrates monocrystalline silicon with SiC through an optically ultra-thin interlayer, we experimentally realize a quasi-BIC thermal emitter with a measured Q-factor of 181. This represents the highest reported Q-factor for thermal emitters operating at wavelengths longer than 7 µm, and establishes photonic-phononic nonlocal metasurfaces as a platform for coherent and spectrally selective mid-infrared thermal radiation.