I'm sure you're much more informed than me about it but it was actually the page you've quoted there that made me wonder:
Indefinite leave to remain for a work permit holder
134. Indefinite leave to remain may be granted on application to a person provided:
(i) he has spent a continuous period of 5 years lawfully in the UK, of which the most recent period must have been spent with leave as a work permit holder (under paragraphs 128 to 133 of these rules), and the remainder must be any combination of leave as a work permit holder or leave as a highly skilled migrant (under paragraphs 135A to 135F of these rules) or leave as a self-employed lawyer (under the concession that appeared in Chapter 6, Section 1 Annex D of the Immigration Directorate Instructions), or leave as a writer, composer or artist (under paragraphs 232 to 237 of these rules);
That doesn't say the whole 5 years have to be on a work permit. It says that it has to be five years continuous and the last part on a work permit. Sorry, not trying to be difficult, just spent hours wading through that today and can't see where I'm going wrong.
Ok, I understood the paragraph now, there's a list beneath of what the previous visas could have been, not including student. I'm an idiot. Ignore me.