The fact that they are equivalent is what makes non-hardened keys
useful (one can derive child public keys of a given parent key without
knowing any private key), and also what distinguishes them from
hardened keys. The reason for not always using non-hardened keys
(which are more useful) is security; see further for more information.
That little apostrophe
' means that branch of the key derivation tree is "hardened" -- and THAT means that you must have the private key to derive its child keys.
So if you have the PRIVATE key for
m/84'/0'/0' you can generate child keys for example
m/84'/0'/0'/0' (these are two different keys, one is hardened the other is not).
If you were to derive the non-hardened key at
m/84'/0'/0'/0 then you could use an xpub FROM THIS KEY (not any of the parent keys) to derive public child keys, without any private keys. Examples would be