In addition they have business accounts which don’t have any of the costly features like interest, free cash deposits, etc so they’re profitable for Starling as they profit off the interest (especially relevant for business accounts as they might hold high balances keeping money aside for taxes and VAT) without giving out many costly features like cash deposits (the only overhead there is support and foreign ATM withdrawals).
Domestic ATM withdrawals are a cost for Starling as they have no network of their own; they're not even part of Link.
I'm not sure that the paltry interest they can gain from business deposits will be enough to make those accounts profitable (they just announced another £60m of funding this week).
I'm a relatively happy personal customer of Starling, so I'm not against them, but I do think it's a bit optimistic to say Starling's business accounts are profitable based on no evidence.