From openDesk website:

> Create, edit and share documents, spreadsheets and presentations with full support for all major file formats

It's missing from the list of their products though :(

https://www.opendesk.eu/en/product

See the Socument Management section from the link you provided

> Create, edit and share documents, spreadsheets and presentations with full support for all major file formats.

Take a look at the Document Management section on that page

> Create, edit and share documents, spreadsheets and presentations with full support for all major file formats.

The document editing portion just uses Collabora which is based on Libreoffice.

The Excel replacement they use is this one: https://www.collaboraonline.com/calc/