They know how much TT you have right? The fact you got the invite anyway means they are willing to consider you. I'd go into it with the mentality that you have every chance at getting the job. If you instead find that they aren't interested because of your low TT you can shift to a message of "I know my TT is low but this is a great networking opportunity and I really do want to work for Spirit as soon as my experience meets your minimums." You at least then have a chance to make a good impression and make getting a future interview easier.