Electric rates are determined from a number of factors, fixed costs, variable costs, profit margin, market demand, and more.

Selling the excess electricity to the Bitcoin mining ops provides revenue such that the organization won't seek to raise rates to cover their costs + profits (or would be denied if they were to attempt to seek an increase).

So yes, adding Bitcoin mining can essentially be a subsidy that either limits rate increases for the other customers, or (less likely) can actually lower their rates.

That's likely what she was inferring.