They did not invent object storage so much as popularize it and standardize the terminology we now use. S3 was announced in 2006. By then, many other distributed object stores had already existed for years, mostly for purely archival use and often a bit enterprise-ish (e.g. FilePool/Centera in 2002). Depending on how much similarity you require, you could even trace lineage back to NASD in 1995. There's also a closely related space of P2P applications which had different goals but similar APIs and implementation details. For example, Freenet and Gnutella both started in 2000. It's practically certain that some techniques worked out in that space informed the design of S3.

Unfortunately Amazon has not done a very good job crediting predecessors, but they do deserve credit for bringing what had previously been rather niche ideas (I know because I was there throughout) to the masses.