The term ‘pre-fortis clipping’ refers not only to the shortening of vowels, but also any sonorants (i.e. approximants or nasals) that may intervene before the fortis obstruent in the coda of the syllable.
The reason given in the blog for the invention of the term pre-fortis clipping is indeed that the previously used term shortening can cause confusion—especially in the context of Southern Standard British English (aka RP)—when discussing vowels. This is because SSBE arguably has two natural classes of vowel already referred to as ‘long’ and ‘short’ vowels. However the term itself has always referred to the shortening of all the sonorant material in the nucleus or coda before the final fortis consonant.
If you look carefully at the excerpt from Wells's paper, you will notice examples of pre-fortis clipping which include approximants, such as /l/, or, implicitly, nasals such as /m/. I have put these in bold below:
This is the name which some of us have come to adopt for the rule making the /el/ of shelf durationally different from the /el/ of shelve, and the /iː/ of feet different from that of feed. (Gimson refers sometimes to ‘shortness’ of the sounds involved, sometimes to ‘reduction’. Calling such sounds ‘short’ leads to confusion when pairs of phonemically distinct vowels such as /iː/ and /ɪ/ are also categorised as ‘long’ and ‘short’ respectively; calling them ‘reduced’ is to be avoided since this term for most phoneticians denotes change of quality, a ‘reduced’ vowel being of the [ə] type. The term ‘clipping’ avoids these difficulties.)
English vowels are subject to pre-fortis clipping, then, when they are followed by a fortis consonant within the same syllable. The /f/’s in self, selfish /ˈself.ɪʃ/, and dolphin /ˈdɒlf.ɪn/ trigger clipping, but not those in shellfish /ˈʃel.fɪʃ/ or funfair /ˈfʌn.feə/. So do the /t/ in feet and the /ʧ/ in feature, but not the /p/ in fee-paying or the /k/ in tea-kettle. The vowel /æ/ undergoes pre-fortis clipping in lap, lamp, happy /ˈhæp.ɪ/, and hamper /ˈhæmp.ə/, but not in slab or clamber. Available here
Here are some examples of descriptions of pre-fortis clipping:
So to answer the Original Poster's question, the standard descriptions and definitions of pre-fortis clipping do indeed include the reduction of not only vowels but also any attendant approximants or nasals.
However, I am unaware of this ever being described as affecting only the sonorant and not the preceding vowel (this does not necessarily mean very much though). I would be very interested to see some actual data in relation the the Original Poster's example.
This excerpt cited in John Wells' blogpost Pre-fortis Clipping is from:
Wells, John. 1990. Syllabification and Allophony. In Susan Ramsaran (ed.), Studies in the pronunciation of English, A commemorative volume in honour of A.C. Gimson, 76–86. London and New York: Routledge.