Yes, you are right.
However, this is more complex.
The 15 hrs training requirement for the conversion is a UK only thing. It is not in JAA. I know for a fact that e.g. Switzerland does not apply it - you "just" sit the JAA exams and turn up for the IR checkride.
So there is no such thing as a "credit" on an ICAO IR -> JAA IR conversion. The whole ICAO IR is carried forward. Only the UK wants you to sit with an instructor for 15hrs.
The fact that most people will need > 15hrs of familiarisation flying, with an instructor who knows the tricks the examiner will try on you, is a separate issue.
Englishal - I'd like to meet your friend who did an FAA IR over a weekend. He must have had some very recent flying, logged or not. It can't be done otherwise; the workload on the checkride is too high to just jump in from any sort of "normal flying".