The page for ～じゃないか includes two example sentences that use ～んじゃないか. This mistakenly gives learners the impression that these two grammar points have the same/similar meaning, when in fact they absolutely do not.
～じゃないか is used for making statements and seeking agreement. (“It’s beautiful, isn’t it?” = “It’s obviously beautiful so I’m sure you agree.”)
～んじゃないか is used for softening statements by presenting them as speculation. (“I think it might rain tomorrow” = “I’m not sure it will rain, but that’s my guess.”)
～じゃないか is strong and used to assert that things are true
～んじゃないか is soft and mainly used to speculate
～じゃないか is also used for things other than making statements, like expressing surprise or pointing out another person’s mistakes.
～んじゃない is never, ever used in those types of situations due to its completely opposite, softer tone.
Bunpro has another page for ～のではないだろうか. You could consider ～んじゃないか a variant of ～のではないだろうか, NOT ～じゃないか.
The example sentences using ～んじゃないか should be removed from the ～じゃないか page. They were a major point of confusion for me and I’m sure they are for many other learners too.