They are giving you a line that, with a little close reasoning, will reveal it.

How will water get behind your liner?
Either your liner is torn and is leaking, or it is ground water.
If it is torn and leaking enough long enough to erode your concrete, you have far bigger problems than calcium deficiency.
If your liner is for a steel in-ground structure, or uses a sand base, calcium is again irrelevant, and you have bigger problems from the leakage.
If the water is ground water, the calcium in the pool water won't affect it either.

Give the. calcium to a friend with a concrete, plaster, or other hard-sided pool.

A pool store that will not take back a sealed chemical probably should not get your business, especially if they cook up a crock the way they did.