The problem of a sudden high-pressure hydrogen jet release into a low-pressure environment is highly important in various industrial and aerospace applications that require a safe storage of pressurized hydrogen. In the present work, numerical simulations are carried out for a sudden hydrogen-jet release from a high-pressure vessel into hydrogen at atmospheric pressure through a slit. Two modeling approaches are extensively compared: 1) Full-scale solution of the entire problem, which includes the high-pressure vessel and the low-pressure hydrogen environment; 2) Simplified solution approach, which includes only the low-pressure hydrogen environment under the assumption of choked-flow conditions at the high-pressure vessel slit. It is shown in the present work that the choked-flow assumption can lead to different results from a full-scale computation. In a full-scale computation, the Mach number is subsonic at the slit exit and a "vena contracta" forms downstream of the slit exit. Both approaches are shown to capture the main flow structures, such as the lead shock, contact surface, and Mach shock. However, in the choked-slit assumption, the barrel shock is detached from the slit-exit edge and the jet expands faster than in the full-scale case.