That's true in general, but conditional severance isn't the same thing as unconditional severance. Conditional severance, depending on the conditions, may well be a quid pro quo. If the money is compensation for e.g. a non-disparagement clause than it isn't also customary two weeks severance.

If such practices are widespread in a particular industry than that industry in practice doesn't provide severance. In those circumstances, it makes sense to withdraw the corresponding practice of giving notice. Or perhaps instead substituting a practice of conditional notice.