> Only way it would increase the stock price is by signaling that the company doesn't need money and can return it.
That's not completely true. If a company has $10bn of excess cash and a market capitalisation of $50bn you cannot simply assume that the business is worth $40bn. The market might be assigning a value to this cash lower than the nominal value (for example because there is a risk that the management will just do some stupid acquisition with the money). And beyond valuation considerations, stock prices are affected by supply and demand: buybacks create some artificial demand.
That's not completely true. If a company has $10bn of excess cash and a market capitalisation of $50bn you cannot simply assume that the business is worth $40bn. The market might be assigning a value to this cash lower than the nominal value (for example because there is a risk that the management will just do some stupid acquisition with the money). And beyond valuation considerations, stock prices are affected by supply and demand: buybacks create some artificial demand.