I can give you guys living example. While I was studying full time in UK, I have worked 40 months 20 hours per week. IRCC counted them total 2 years 3.3 months (27 months) foriegn work experience. This means any number of part time work is accepted, it does not limit to 15 hours per week. I have got the information from my own GCMS notes, so no doubt at all. Here
@DelPiero07 is absolutely correct. Initially he helped me to understand this and he is always right about his comments.
Also as per IRCC website 30 hours per week counted as full time work hence the case analyst said it is full time work experience. I think problem here is about NOC selection or not matching duties. If
@MajdB is confident about his work letter then I think VO will accept the letter.