Turbo prop overhaul requirements

I have a question about tbo for FAR 91 turboprop operators, PT6.  I know that tbo is 3600HR and if FAR 135 will need MORE STC to extend past that but part 91 doesn't. 

If we look at TCDS for PT6- ?? E4EA under Note 14 it lists time limited part SB and overhaul SB, so if the engines are not on an extention program how can they be flown past tbo and be up to date with TCDS.??

Also AC 120-113 best practices for engine time in service

section 6 lists out all parts and 6.6.2 states that part 91 operators need to follow maintenance program for turbo prop.  There is a note at the end that states owners of recip engine under 91 don't have to follow recommended TBO.

I have a customer who would like to buy a turboprop that is part 91 and has approx 4000hr since overhaul and 500 hrs since hot section.  For what I am reading the engines need to be on extention STC or overhaul.

Anyone else in same boat? FAA has not been much help.