The thought with the Interest Investment is there exists a large market of investors in LATAM (and emerging markets), however they avoid startups because of a lack of traditional yearly returns, it's more unpredictable, there are unfamiliar terms, etc. Many people have tried to solve this problem by educating investors (to create new angel investors), but the success of this has been questionable. I felt this new idea could bridge this very large gap.
About the 15% stake, you are right that it's not a very liquid market at all. The thought was, if a person invested in a cash-flow profitable startup with good growth rates, there is some value in that asset (this type of investment wouldn't work for an unprofitable startup or a company with zero cash flow). There is a smaller market for private companies, but an investor should put money into an entrepreneur with a good reputation who stays true to his or her word (just like they would with convertible debt).