I wonder if they employ psychoacoustic tricks such as ones described in this paper.
https://www.researchgate.net/public...ual_Bass_System_with_a_Phase-Vocoder_Approach
Abstract (with my emphasis):
A bass enhancement technique based on a phase-vocoder approach is presented. Instead of direct bass boosting, the proposed method creates a bass impression by exploiting the psychoacoustic properties of humans. This technique is most useful in audio reproduction using
small loudspeakers that have no low-frequency capability, where direct boosting will likely result in nonlinear distortions. In light of psychoacoustics, the bass effect is synthesized by augmenting the original signals with high-frequency harmonics. Unlike conventional methods that rely on nonlinear processing, the proposed method performs the required frequency transformation by using a phase-vocoder approach. Apart from frequency transformation, another key element of the proposed technique is the magnitude adjustment of the generated
harmonics. The underlying principle for magnitude adjustment is based on a polynomial model of equal-loudness contours. The method is implemented on a digital signal processor with the aid of multi-rate signal processing. To validate the proposed technique, objective and
subjective experiments are conducted for PC multimedia loudspeakers and handset loudspeakers. The subjective listening experiment followed the procedure of multi-stimuli with the hidden reference and anchor (MUSHRA), and the data were analyzed by using the multi-analysis of variance (MANOVA) method. As indicated by the results, the proposed technique proved effective in rendering bass impression with acceptable audio quality.