Short answer: If you're a customer of both these tools, yes, it's terrific news for you! Otherwise, there are more flexible and cheaper solutions than LookML to create a semantic layer for Tableau.
OLAP is almost 30 years old and has often been used in different contexts over the last 10 years. It's a huge market, and there are many enterprise software in the space. OLAP is simply pre-aggregated data from your raw data, and it's not for everyone. If you're working for a startup that doesn't have terabytes of data, you probably don't need OLAP because you can efficiently run ad-hoc queries on your raw data (also known as fact tables). However, when you have petabytes of data and have tens of people who rely on it to make decisions, you don't want to make them wait 10 minutes for a simple query. Let's breakdown the OLAP into two categories: