https://www.ninerpedia.org/wiki/Programming#Useful_CALL_LOAD...
But you are correct, the base unit didn’t have access to these functions. You did need to get the TI Extended Basic cartridge.
https://atariage.com/forums/topic/218904-playground/
It was possible BITD too, but no one thought hard enough about how to do it. There has been a lot of "pushing the hardware" over the last decade. Lots of things that were thought of as "not possible" have been done now. The 99/4A community is very active, and there is a lot more going on than just new games (although that is also a big part of the activity).