Why switching cause EMI & noise problem?
EMI in power supplies can come from multiple sources, including:


Capacitive currents driven by switching voltages
E-fields produced by switching voltages
Stray H-fields from high di/dt loops and inductors/transformers
High-frequency rectification


The article cites three classifications of converters:


Linear regulators
Charge pumps
Inductive switcher


Linear regulators are step-down converters (input higher than output) which work by dissipating power to achieve regulation, usually in a non-saturated series pass element like a transistor or MOSFET. There is no high-frequency switching going on, so the circuit from an EMI perspective is incredibly quiet.

Charge pumps are switched-capacitor sources, where energy is stored in a capacitor, the capacitor is disconnected from the source and reconnected quickly in such a way to provide a 'boost' of voltage. There is switching going on but in general, they are for extremely low power levels (a few watts at most) and generally are extremely compact in terms of physical layout, so they don't contribute much to EMI (when done well).

What the article means by 'inductive switcher' is a switching power supply that uses inductors as energy storage devices, which comprises the vast majority of power supplies commonly available today. Buck converters, boost converters, flyback converters, half-bridge and full-bridge converters, Cuks and SEPICs - these are all "inductive switchers" and all involve converting DC into high-frequency AC, manipulating the average value (through duty cycle and other means) then rectifying the AC back into DC. You'll see all of my previously cited EMI sources come into play in these converters.

The science behind EMI is incredibly complex, and takes a large amount of theoretical knowledge and 'hands-on' experience to really grasp. It's not something that can be explained properly in a few paragraphs. Just keep in mind that any high-frequency switching (10s of kilohertz or higher) will likely induce EMI that will need to be dealt with somehow, and (in general) the higher the power level, the worse the EMI can be. EMI is also largely influenced by parasitic elements and physical component placement - a bad layout for a 10W converter can cause bigger EMI problems than a nicely-done 100W converter.