SEP is a class of aeroplane.
PA46 & Cirrus fall into the similar SEL class on a US certificate.
It matters not what you, or I, think about the validity of those letters. Nor even the CAA’s opinion.
We are referring to N-registered aircraft: FAA and CFRs apply, and these interpretation letters set the precedent of the FAA’s determination of whether one is legal to fly.
The 61.75 certificate states that is “issued on the basis of” and must be “accompanied by”.
This becomes a risk management decision should one choose to fly an N-registered aircraft outside the USA.