In Jain dharma, the rule is not only about whether a visible network is destroyed. The main concern is ahimsa: how much violence is caused to living beings.

So even if one argues that mycelium may remain, mushrooms are still generally avoided in Jain tradition because they are considered to involve subtle life and growth in damp, decomposing, and organism-rich conditions.

Also, your comparison is not sufficient from a Jain view. Some foods Jains avoid cause much greater harm, especially:

• Roots like potato, onion, garlic, carrot, radish, etc., because uprooting them destroys the whole plant and many subtle lives may be affected.
• Fresh sprouting and heavily disturbed foods can also involve more violence than simple above-ground fruits or grains.

So the Jain principle is not “which one looks more damaged,” but which choice causes less violence overall.

In short: Jains avoid mushrooms not because the mycelium is ignored, but because the food is not considered acceptable within ahimsa-based discipline.