Large uploads via /get_temporary_upload_link are terminated with '413 Request Entity Too Large'

Unfortunately, the links returned by get_temporary_upload_link do have a file size limit, as you found. Apologies this isn't better documented! I'll ask the team to clarify this.

There isn't an alternative though, so that does mean that in this case, to upload large files, you'd need to either give the access token to the client to perform the upload session calls client-side (which isn't recommended for untrusted clients, of course), or send all of the data to upload through your server to perform the upload session calls there.

I'll also send this along as a feature request for a 'get_temporary_upload_link'-like version of upload sessions to support uploading large files without direct access to the access token, but I can't promise if or when that would be implemented.