If you use IRQF_DISABLED, then your interrupt handler is guaranteed to be called with interrupts disabled. If you don't use IRQF_DISABLED, then officially it is implementation-specific whether your interrupt handler is called with interrupts enabled or disabled. It is apparent from the rest of the article that the current implementation-specific behavior depends on CONFIG_LOCKDEP.
The important thing to remember when writing interrupt handlers is to use e.g. spin_lock_irqsave() instead of spin_lock_irq() (assuming you need a irq-disabling variant; see the Unreliable Guide To Locking for more details). Then it will work with CONFIG_LOCKDEP=y and CONFIG_LOCKDEP=n and on a wide variety of kernel versions.