I have a background in several scientific technologies so what I would consider to be significant data based on valid “case studies” may be much more demanding than all of the SEO “evidence” I have ever seen. I have yet to see any SEO evidence that convinced me that what Google representatives themselves have said is not correct.
Anyway, can an app be in a sub folder of another app? In many cases, sure. Potential pitfalls that come to mind are:
- HTTP routing issues
- filesystem path issues
- naming clashes
- dependency incompatibilities
When App1 and App2 are separate, if there is a problem with one the other will be unaffected. But when App1 and App2 are not separate, a problem in either can break both.