You’ve misunderstood. No one said that they were equivalent, only that there was no functional difference between them. They certainly allow you to access different characters, but whether you hold down shift and then type a “d” to input a capital “D” or you tap shift and then tap the “d” to get the same result is merely a software setting that you can toggle. The same is true for AltGr and compose keys. You can configure your OS to require them to be typed as chords or as dead keys, whichever you prefer.

See also the space–cadet keyboard (<https://en.wikipedia.org/wiki/Space-cadet_keyboard#/media/Fi...>), which had no fewer than seven such keys labeled control, meta, super, hyper, shift, front, and top.