Did Leonardo da Vinci actually invent the things attributed to him?
He designed a great many things and built almost none of them, and several of the most famous attributions are either not his ideas or would not have worked. The gap between designing and inventing is the whole issue.
What the notebooks actually contain. Thousands of pages of sketches, observations and speculations, largely unpublished in his lifetime and written in mirror script — probably for convenience as a left-hander rather than for secrecy. Because they were dispersed and not printed for centuries, they influenced nobody, which matters for any claim that he invented something: an invention that reaches no one does not enter the historical chain.
The famous attributions, examined:
The helicopter. His aerial screw is a genuine and striking design. It also could not have flown — no material of the period was strong and light enough, and a single rotor would have spun the craft rather than lifting it. It is a beautiful conceptual sketch, not a working machine.
The parachute. His pyramid design has been built and flown successfully in modern tests, which is a real vindication. But similar concepts appear in earlier manuscripts, so the idea was not solely his.
The tank. An armoured vehicle drawing exists — with gearing drawn incorrectly, such that the wheels would have turned in opposite directions. Whether this was error or deliberate is argued.
The bicycle. A sketch in the Codex Atlanticus is now widely regarded as a twentieth-century forgery added during restoration.
The flying machines, based on flapping wings, could not have worked given human power-to-weight ratios.
Where his achievement is genuine and understated: anatomical drawing of extraordinary accuracy, based on dissection, including the first correct depiction of several structures; observations on fluid dynamics, geology and optics that anticipated later findings; and the paintings, which are not in doubt.
The honest summary: a superb observer and draughtsman whose speculative engineering is frequently overstated, and whose scientific work was genuinely ahead of its time and largely lost to it.