Welcome to the community!
Indeed, Coda is not an app-building platform; you cannot build packageable apps with it. The “Coda apps” statement is just a marketing copy, just like the “No code” one (because formulas are technically still code, just not the “conventional programming language” kind of code like C++ or Python).
What Coda actually is — a cloud doc platform, think Excel online / Google Sheets, but with more ways to present the data, and with buttons that you can program to perform actions.
I believe, the “doc as powerful as an app” statement is there only to set Coda apart from rivals like Notion. Coda is basically Notion + actions. This addition lets you build custom workflows, not only to-dos and journals.
To sum up:
Coda does not let you make apps — but it lets you build docs that can substitute many kinds of apps.
Then there’s also me who likes to abuse Coda and build various crazy stuff, like a chat bot or this snake game, just for fun and to explore the potential of the platform