It's a strange definition, but now I'll understand what you're writing about I suppose.
Interjections (and sometimes particles) cause trouble for folks but they don't usually get excluded from language on those grounds. Words like tsk-tsk (actually two avleolar clicks) have meaning and can be used inappropriately. If I utter ha ha when somebody burns himself, I should probably be prepared for a bit of trouble. Same if I say ouch when somebody tells me they've just named their baby "Jane". To me that's semantics and pragmatics.
The whole parts of speech concept is problematic, but the word "word" has a certain meaning vis-a-vis language, and I don't think we should exclude words like hmmm or d'oh from the language just because they don't fit into our linguistic framework. Linguists have traditionally had problems with the left over bits that don't fit into their current theory du jour. A great book that discusses this problem is Jean-Jacques LeCercle's The Violence of Language and to a lesser extent his Philosophy of Nonsense: The Intuitions of Victorian Nonsense Literature. OTOH, some linguists are interested in these left-over bits. Glossolalia is one interesting phenomenon where semantics and words part ways.
One interesting thing about interjections is that new pronunciations have come about because of their spellings, e.g., tsk-tsk being pronounced by some as tisk-tisk.
