Is Zac still a tank, or can he be considered an AP bruiser?

Zac is different from many other tanks. Most tanks have an offensive spell that scale with a defensive stat. For example, Malphite has his E scaling with armor, Sejuani has her W, which does damage based on her health.

However, Zac himself does not have any damage abilities scaling with defensive stats that he can build. Some tanks like Poppy and Maokai do not have that either, but at least they have some form of defensive steroid, and their CC is much more reliable, which is not the case with Zac. He has that passive which provides sustain, but it is extremely unreliable, and enemies can easily deny it by stepping on the blobs.

Because of this I've found that building Zac as a full tank does not work as well, unlike the aforementioned tanks. However, he really works well with a build that stacks as much magic penetration as possible with items that provide him defense. Something like - Zac's AP ratios are below average, but the main reason to stack magic penetration is because his bases are very high and his W hurts with magic penetration; also, it also lets Sunfire Cape hurts even more than with other tanks.

Can at this point, Zac still be considered a tank? Or he is different from the others?