I agree the optimization looks backwards to me - in most of the microcontrollers I work on, the flash program space can easily be multiple megabytes, but the ram is usually the limiter around 128-512k.
The use case which has motivated our work is code mobility where an executable program can be embedded in a document, email, or website. In that use case the size of the program must be small to minimize the transmission or loading time or to satisfy space constraints, such as the size of a disk boot sector, the URL length limit and the UDP packet size. On the other hand, the space used while the program is executing is of secondary importance.
So the main target application is not microcontrollers, even if it can obviously be used for some microcontrollers (for example a 1-2$ ESP32-C3 has 400 KB of RAM).
One of the possible applications of Ribbit is to implement the RVM (Ribbit VM) in the Excel spreadsheet formula language (which will soon be turing-complete with the addition of the LAMBDA form) to be able to program spreadsheets in Scheme. The execution environment has lots and lots of RAM, but you want the .xls file itself (which contains the RVM) to be as small as possible so it can easily be sent in emails, etc.
If you are interested in building a a self-contained HTML document that embeds a full featured Scheme implementation, you should look into Gambit. Check out https://try.gambitscheme.org