Hello! My name is Humberto Valle, I'm a marketing strategist for www.unthink.me. The term “growth hacking” is now being widely used to describe startup marketing, but its meaning has changed since Sean Ellis originally coined the term in his 2010 blog post. You have a very good question here! In short, the answer is No, its not just for SaaS. Service based companies, professionals looking for work, and other non-digital physical product based companies use Growth Hack methodologies every day with great success.
Let's get a few things straight first, there is no traditional or digital marketing, only good marketing. Just as how there is a difference in advertising and marketing. Marketing is a management level effort for anything customer facing while advertising is much much simpler than that, albeit still a complex responsibility.
traditional is now also including platforms like social media and PPC so in many ways, growth hackers are referred to as that when they can combine management level insight in a business and it's market and pair with easily launched and measured customer acquisition tactics. Sometimes these efforts can very well combine traditional and new traditional as well as highly technical efforts.

What differentiates a growth hacker from a "traditional" marketer? Well, while a growth hacker certainly has the skills of what is now becoming traditional marketing in his toolkit, Sean Ellis recognizes that those skills are not always relevant to most early startups, but just as well they are used by many.

I have personally implemented non-technical growth hacking methodologies for service-based businesses such as in the cleaning industry, manufacturing of sports apparel, table top games, and others. I haven't had the chance yet to apply Growth Hacking to a SaaS which tells me that it can be done outside of the SaaS industry. If you think about it, growth hacking is understanding where your customers are coming from, what will make them tick, and how to be in the right place at the right time with an offer that is so easy to claim or engage with that it's almost intuitive so the conversion is high. Growth Hacking may have been coined by a technical aspect for a SaaS but in reality is no more than good practice marketing efforts at a management level effort.
As a business owner, startup founder, or high level CEO - and of course as head of marketing - you must know who your ideal target it, the market, the competitors, things that make your prospects tick, how much are they willing to go through to acquire what you offer, what are their stages of decision making, howlong is your pipeline for conversion, where and how to retarget them, etc. - this is what every good marketer should be able to do, but in many ways it was unfortunately coined for the higher level experts like us who do have this type of insight when it reality it should be common practice for anyone wanting to call her or himself a 'marketer.'