spectral leakage for integral number of periods
If you have an integer number of cycles (exactly) within a DFT frame, and the signal is not windowed (aside from the rectangular window extending to the edges of the DFT fram that would result), there is no spectral leakage. What the article is referring to I believe is more commonly called zero padding which would then cause the DFT to approach the DTFT, interpolating frequency samples between the bins. I personally wouldn't call this spectral leakage although you could mathematically get to the same conclusion in that energy in a DFT bin simply represents correlation to the input signal. If energy is there, and the bin isn't your true frequency, then sure, it is "spectral leakage". In that paradigm however of zero-padding, I prefer to associate it with the concept of frequency interpolation (again the math and the results are the same, so no disagreement - just convention).

Spectral leakage occurs when you have a non-integer number of cycles such that your true frequency is mid way between DFT bins. Each DFT bin actually has a very wide frequency response with nulls at every other bin. So a frequency that is in between bins will show up within the response of the other bins, while a frequency that is exactly on bin center (integer number of cycles) will be in the null of the responses of the other bins. There is therefore no leakage under this condition (that is not an illusion as the article suggests- there is simply no leakage - but I do see how the author presented both options and stated "depending on your point of view"). And for the other conditions the signal "leaks" into the response of the other bins.

I explain this further here at this post.

Intuition for sidelobes in FFT