There is certainly some overlap between the two projects. The WebGL build target supports most of the features Unity ships with. The trade off is that the resulting projects are large which result in longer transport and startup times. If your goal is to publish more complex web-based simulations that run on desktop browsers, this is a strong option.
Project Tiny is a much lighter approach that provides very fast load times and small delivery size. This is achieved by providing fine grain control over what code modules are included. Project Tiny is designed for mobile viability which is why the initial release is recommended for Playable Ads and Games that run in messaging apps or social media platforms.