I think the 'self-education' argument arises from the specific 'You may not' for "use the programs to provide third party training;". It explicitly states third-party, rather than training/education in general.
It can be argued that 'development of a prototype' necessarily requires (or should) an understanding of the database functionality and that self-education is therefore a preliminary stage of development. Pre 'XE', this was probably a stronger argument. With a more liberal 'XE' license available, this is a better option for self-education (or third-party training) as long as you don't need the Enterprise Edition components.