Context. The origin of the highly ionized iron emission (Fe XXV at 6.7 keV) characterizing Galactic ridge X-ray emission (GRXE) remains a fundamental piece of the puzzle in high-energy astrophysics. Although the GRXE continuum was largely resolved into discrete populations of cataclysmic variables and coronally active stars, subsequent measurements have revealed that these sources exhibit Fe XXV equivalent widths that are significantly lower than that of the total GRXE, leaving the intense iron line emission unexplained. Aims. We aim to identify and characterize hard X-ray sources (>2 keV) within the XMM-Newton heritage survey of the inner Galactic disk that possess reliable Gaia counterparts. By analyzing their X-ray spectra and the presence of highly ionized iron emission lines, we seek to determine the physical nature of these hard X-ray emitters among the late-type giant population and evaluate their collective contribution to the Galactic X-ray background. Methods. We cross-correlated the XMM-Newton survey of the inner Galactic disk with Gaia DR3 astrometry. Sources located within the red giant branch of the color-magnitude diagram were selected for the X-ray spectral analysis based on their significant hard X-ray emission. We derived their X-ray luminosities (LX) and spectral hardness ratios, with a particular focus on identifying signatures of extremely hot plasma; notably the Fe XXV line complex at ∼6.7 keV. Whenever possible, the optically variable nature of these sources was verified against established long-period variable identifications. Results. We identified 107 X-ray sources consistent with the long-period variable populations in the Gaia color-magnitude diagram. These sources exhibit high X-ray luminosities (LX ≍ 1031−1033 erg s−1), notably exceeding the typical saturation levels of single giants. Their X-ray spectra are significantly harder than those of quiescent stellar coronae, with plasma temperatures reaching up to kT ≍ 6 keV adding a prominent emission feature at ~6.1 keV. Conclusions. The combination of high LX, hard X-ray spectra, and prominent 6.7 keV Fe XXV emission indicates these sources are a distinct population of accretion-powered binaries associated with late-type giants. Our analysis shows this population contributes ∼20% of the total GRXE continuum and ∼40% of its iron line emission.