You do realize that is utter and complete hogwash, don't you?
Yeah, the NWS issues convective SIGMETS just for the fun of it. Nothing to worry about, just fly right through those thunderstorms. There are a number of meteorological phenomena that can threaten the structural integrity of an aircraft, or cause loss of control, or both (often the second followed by the first). Aside from thunderstorms, the rotor under a strong mountain wave is capable of swatting any aircraft type from the sky if it’s strong enough, as just one additional example.
Of course, there are exceptions, such as hurricane hunter C-130s, but they don’t just plow straight into a hurricane. If they did, they would most likely also experience a rapid unplanned disassembly. These aircraft fly carefully planned entry and exit paths designed to gradually spiral into the bands of the storm by flying with the direction of rotation to reduce the effect of the high winds and avoiding the most intense embedded thunderstorms. As for normal civilian and even military aircraft, they avoid flying through thunderstorms when at all possible. If you think otherwise, you’re going to have to cite the normal everyday military aircraft (not uniquely designed and operated research flights) that just ignores thunderstorms and plows right through them on a routine basis. Hint: There is no such aircraft. As the original poster said, EVERY aircraft type in existence is highly vulnerable to destruction or loss of control if it ventures directly into a strong enough thunderstorm.