This is probably true, but remember that the single inventor has the option of publishing the invention rather than filing a patent application. The publication will then become prior art to any later filed patent application by another. The inventor may then file for a patent on the invention within a year of publication, but may have to overcome significant prior art that has arisen in intervening time (i.e., between publication and filing). For a number of reasons (that I'm not going into here) this may not be the best strategy in most, or possibly any, situations, but it does exist.
