Difference between alert log and archive log in oracle databases
Alert log is a database log file where you can find various messages about your database, like time of startup, some db parameters, log file switch, possible errors that can occur etc.

Archive logs are archived redo(online) log files. In redo log files are all changes that happened to your data. If your database is in archivelog mode, than redo logs can't be overwritten, they are archived in some other location when they are full. Redo and archive logs are used in various database recovery scenarios.

So your Oracle expert probably used LogMiner(or some similar tool) to search through archive log files to find something like truncate table some_table;