That's very well possible.
There are very many variables to that quote you quoted.
someone can come to your site, inject code into your database..... then leave your site..... someone else can come to your site, fill out a form or such, but get redirected to somewhere else....... all because the culprits have already injected the code into your database.
Trust me on this... there are more than one way to inject SQL injection onto you, but if your site is well setup, and your database is well configured,, meaning your php, mysql, or whatever you use, then you should be fine. Nothing is guaranteed, and nothing can keep you 100% safe, except the knowledge that you receive. You'll be 100% safe with knowledge.
And if I were you, I'd take the time to research this on many other sites.... and not just one site. Personally, I find that odd for a person to want to know about how people inject poison into others, but yet and still, don't want to read the medical information on it, just so you can reduce the chance of it being injected into you.
I'm encouraging you to do some research on this.... it's not going to take you your whole life time, ...but at the same time, it will better prepare you for your web site ventures.
If you're really concerned about this, don't wait until you become a victim. Go find out., and post back here what you've learned, so others can learn from you and your experiences in combating this. Do what you can to help yourself, and then take that to help others too.
Just think about it..... BleepingComputer is helping you right now.....
Go get'em tiger.........