True Linking Verbs

I've read it on a website that "True Linking Verbs, as their name implies, are always linking verbs and any form of verb "to be" is a linking verb."

But is it necessary for a linking verb to be the main verb of the object.

Consider the following example;

1. He is going to school.

Here "is" is an auxiliary.

So can we call it a linking verb?

GB