That means, in short, all bitcoin client releases were not backward compatible in nature. Does not it lead to the fact that Bitcoin has hard forked before?
No, Bitcoin has not hard forked before, and all releases are actually backwards compatible since all protocol changes have been soft forks. As 2112 said earlier, you can actually get past the BDB lock issue with some special configuration. So in theory, you could sync a 0.1.0 client fully up to date with some special configuration.