I think you misinterpreted the last example you linked - I don't think there's anything about 9 being a suspension.
sus9 is a very popular chord - it's a suspended chord with a dominant 7th and 9th. Gsus9 would be G C D F A. Since D does not contribute to the sound, it's usually played as G C F A, which is often notated as F/G.
It sounds really nice when you have a cadence and want to finish on a dominant, but "ligher". Andrew Lloyd Webber uses it in half of his songs... it's the repeated chord in "Mr Mistoffelees", for example.
From the practical perspective, the idea of the sus chord can be distilled to "a major or dominant chord constructed so that the natural 4th is not a dissonance". It does not matter whether the third is somewhere in the chord, or whether you add natural 2 or not.
Actually, calling it a suspension or retardation is silly, since in modern music the sound does not resolve... so actually it's neither.
Half of the problems with the question and its answers is that people are mixing historical, prescriptive harmony of the early 19th century and modern-ish chord notation. Both are interesting and worth knowing, but the usual approach of applying the first to explain the latter does not work.
Addendum (after reading other answers and comments)
In the classical theory, suspensions are DEFINED by the fact that they resolve. If a bar begins with a C/G chord and then continues with G and ends with C, then the first chord is not "C" (even though it looks like C), but rather "G with suspended 3 and 5" or "G with C instead of B and E instead of D".
Which is why if someone claims to use "the real, classic music theory" in the context of a single chord, they are just being pretentious.
In modern music on the other hand - sus chords seldom, if ever, resolve. "Resolving" sus to major gives a very distinct "medieval", "church-like" sound (and begs for sus4 -> sus2 -> major). So yes, the name "sus" might come from the original classic idea of "suspending", but nowadays it's just a historical foot note.
So, what's the point of sus chords? Currently, there's no single music theory that works across genres and "explains stuff". It's mostly hand-waving and acting smug. But if you just take the common part of the "modern" theory and practice (on comping instruments, like piano and guitar), you might picture it like this:
Generally when we play a major or a dominant chord, ALL the sounds from the relevant major scale sound will "work well", except for the fourth (eleventh). In practice, when a pianist sees a C major chord and needs to play the chord with the left hand, they use multiple voicings: C-E-G, C-E-B, E-G-A-D (assuming the bassist is playing C), E-A-D, B-D-E-G... they all "work" for modern ears under most circumstances. There's just one note that would not work in the left hand: F.
It's the same for C7: default piano voicings include C-E-Bb, Bb-D-E-A, E-A-Bb-D. There's just one note from the C mixolydian that doesn't quite work with the C7 chord: F, the fourth note of the scale.
This is why you will generally not find 11 chords in the wild. When 11 is mentioned, it is usualy altered (#11 is nice).
A "sus" chord is a chord with the third removed (usually) or placed so that it doesn't clash with the fourth/eleventh (for example - high above). It has the interesting property of working like a dominant ("suspends" the tonic), but it is cononant with every note of the scale.
If you take this perspective, sus2 is not interesting... you can use 2 freely anyway, with any C major chord, including sus4. sus4 means: "we removed the source of dissonance from this chord, feel free to play any note of the scale over it".