Does your taijiquan qualify as 'kung fu'?

This is a good question.
It really depends on how the art is practiced and how much time you commit to practice.

Although most taijiquan students train kung fu skills, they cannot honestly claim to be a kung fu student.
Kung fu literally means 'hard work'.
A kung fu student attends class 2-3 times a week and trains anywhere between 1-4 hours a day at home.
This may be regarded as a serious commitment to gaining and refining martial skill.

Most taijiquan students seek a milder degree of commitment; perhaps training once a week in class and maybe doing a little training at home.
They are likely to gain credible and effective self defence skills, but they are not demonstrating a kung fu approach to training.