This
https://github.com/enonic/starter-react4xp/blob/16a350485827d80be28aea384c3367d251edac5b/package.json#L94

Runs this:
https://github.com/enonic/npm-react4xp/blob/5911b76ea605c5595148390f6054dc11b781969c/package.json#L114

Which finally runs this:
https://github.com/enonic/npm-react4xp/blob/5911b76ea605c5595148390f6054dc11b781969c/package.json#L127

Which is:

npx tsup --config tsup.config.client.ts

Which will give this error:

npx tsup --config tsup.config.client.ts
✘ [ERROR] The entry point "/private/tmp/myproject/node_modules/@enonic/react4xp/tsup.config.client.ts" cannot be marked as external

Error: Build failed with 1 error:
error: The entry point "/private/tmp/myproject/node_modules/@enonic/react4xp/tsup.config.client.ts" cannot be marked as external
    at failureErrorWithLog (/private/tmp/myproject/node_modules/@enonic/react4xp/node_modules/esbuild/lib/main.js:1651:15)
    at /private/tmp/myproject/node_modules/@enonic/react4xp/node_modules/esbuild/lib/main.js:1059:25
    at runOnEndCallbacks (/private/tmp/myproject/node_modules/@enonic/react4xp/node_modules/esbuild/lib/main.js:1486:45)
    at buildResponseToResult (/private/tmp/myproject/node_modules/@enonic/react4xp/node_modules/esbuild/lib/main.js:1057:7)
    at /private/tmp/myproject/node_modules/@enonic/react4xp/node_modules/esbuild/lib/main.js:1086:16
    at responseCallbacks.<computed> (/private/tmp/myproject/node_modules/@enonic/react4xp/node_modules/esbuild/lib/main.js:704:9)
    at handleIncomingPacket (/private/tmp/myproject/node_modules/@enonic/react4xp/node_modules/esbuild/lib/main.js:764:9)
    at Socket.readFromStdout (/private/tmp/myproject/node_modules/@enonic/react4xp/node_modules/esbuild/lib/main.js:680:7)
    at Socket.emit (node:events:518:28)
    at addChunk (node:internal/streams/readable:559:12)

Editing tsup.config.client.ts changes nothing, it seem's it doesn't even attempt to read the file.

If I rename /private/tmp/myproject/node_modules to /private/tmp/myproject/whatever

Then cd /private/tmp/myproject/whatever/@enonic/react4xp/
I can run npm install
And now npx tsup --config tsup.config.client.ts works.

So it seems to mark everything under "node_modules" as external?