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Properties of the solar radio spectrum, as well as the temporal profiles of flare emission, indicate the thermal nature of the sub-terahertz (sub-THz) component observed as the growth of radio emission in the frequency range of 100-1000 GHz. The sub-THz flare onset can be ahead of the impulsive phase for several minutes. However, the origin of the pre-impulsive and impulsive sub-THz emission remains unclear. The present work is devoted to a detailed analysis of the M4.0 X-class solar flare observed on March 28, 2022 with the Bauman Moscow State Technical University Radio Telescope RT-7.5 at 93 GHz. We supply these data with multiwavelength solar observations in the X-ray (GOES, GBM/Fermi), extreme ultraviolet (AIA/SDO), and microwave ranges. The differential emission measure (DEM) responsible for EUV emission is determined by solving the inverse problem based on the AIA/SDO data. Using the DEM and assuming a thermal free-free emission mechanism in pre-impulsive and impulsive phases, we calculated the millimeter emission flux of coronal plasma of the flare source, which turned out to be much smaller than the observed values. We concluded that electrons accelerated in the corona and heat fluxes from the coronal loop top cannot be responsible for heating the sub-THz emission source located in the transition region and upper chromosphere. A possible origin of chromospheric heating in the pre-impulsive phase of the solar flare is discussed., Comment: 7 pages, 4 figures, Geomagnetism and Aeronomy, accepted