You need to differenciate between the pieces an extenension enables. They can bring in syntax highlighting through tree-sitter as well as additional language servers. The elixir extension does include heex tree sitter grammer. Autocomplete comes from LSPs. For html files zed runs the built in html lsp. That one wouldn’t understand heex though. For the elixir LSPs available the support for heex is wip at best and imo their maintainers focus is better used improving their support of elixir code before their support of heex.
In the end heex is not html. This is not as simple as “just do what you do for html for this other file”.
