Hello DacrimL,
An interesting story. Was the article referring to any specific subject type? That may be good advice for a passive subject (landscape etc) but I wouldn't think it is applicable for shooting kids, motorsport or other subjects that are moving rapidly, if you want a sharp image.
Regarding your question on the focal length magnification (also known as 'crop factor'), this should be readily found by searching on that subject and including your camera make/model.
The crop factor is simply a ratio of the size of a full frame sensor (35mm film) to the size of the sensor in your camera. Why it can change the effective focal length of your lens is probably explained in great detail with diagrams, on a number of web sites...and probably also on this site somewhere.

Cheers,
Terry