In response to Your main answer "Is Filemaker a web platform?" i would says no.
Filemaker it's a database engine with gui facilities, so You can build things fast, it's an ideal tool for making projects (more or less specific) inside an corp or organization or to prototype/develop an application.
Filemaker has btw at least to modes/tools to share data and gui functions out of the box (http://www.filemaker.com/products/filemaker-pro/web-publishing.html):
Instant web publishing avaiable in the Filemaker Pro (limited users and functionality) and Filemaker Server that enable You and Your group to share with minimum effort data and gui (forms)
Custom Web Publishing only with Filemaker servershare data trough PHP code, with some tools to auto-generate PHP code.
So Filemaker i would says can share you data and forms build inside Your database over the web and can integrate with the Server version with PHP.
Good question btw! Rob