Yes, in superlative form, there is no reference. But in your example, de la classe isn’t a reference but a restriction. You can also say:

“Sophie est la plus grande.”

It’s still in superlative form.

• You can spot a comparative form when the preposition que is present and introducing a reference.
• You can spot a superlative form when an article (le, la or les) is present just before the comparison. In your example, it’s la (la plus grande).

The preposition de (or others) introducing a restriction is optional and can be present in comparative form too:

“Sophie est plus grande que les autres dans ce groupe.”