IMDB is a user-submitted database. Anyone can contribute. I am a regular contributor, usually adding info or making corrections on people I know or projects I'm familiar with. There are tons of projects people have made or worked on that are not in IMDB. Not every filmmaker submits their projects for inclusion, and IMDB doesn't accept every submission.
A title has to meet certain criteria - a biggie is where it was first exhibited. If it didn't play somewhere with admission available to the general public, or wasn't in one of the festivals they accept, it won't go into the database unless you can make a case for it and substantiate your argument. For example, they don't usually accept student films or anything made for the 48HFP unless the filmmaker or someone in the cast has a body of work already in the database or has become a "public figure." Films that played in festivals are only accepted if they were juried selections. Of course, many exceptions to the rules have been made in the past, and will continue to be made -- but no, don't count on IMDB as a primary source of reliable information.
Regarding the filmmaker you cited, Erez Tadmor, IMDB shows that he's directed seven films in the last ten years, three of which were features and most of which he wrote or co-wrote, and has a project in development now - why does that seem like just a few to you?