I mean yeah, it doesn't aim to be a "cross-platform compilation/building system" so of course dependencies is up to you to solve, AFAIK AppImage only aims to solve packaging itself, not what goes into that package.
Which doesn't solve the same problem that Flatpak solves, namely having a package format that a developer can target and it run the same everywhere.
Which doesn't solve the same problem that Flatpak solves, namely having a package format that a developer can target and it run the same everywhere.