Free software, as defined by the fsf, preserves the users freedom to study, change and distribute the software. The right to study entails that the source code be made available: Wikipedia article. Calibre is licensed as GPL3, thus it is free software as defined by the fsf.

Open source is a subset of free software. Free software is always open source.

both GNU emacs and vim can have autocompletion powered by the same language servers that vscode uses. They support the same features (jump to definition, rename symbol under cursor etc etc) as well.

Calibre is free software