They probably have a lot of loans underwritten by stock, so they need their stock price to stay at certain numbers or else their credit rating gets worse and they have to pay more to service their debt.
Unless you are a car company that takes pre-orders, people only pay for cars after they are made. So you either need to hoard cash or take on debt in order to maintain consistent cash flow.
Issuing and buying back shares is a pretty common way to handle cash flow and is really a separate issue to living wages and employee compensation.
Their buyback plan was actually kicked off in 2022. They temporarily paused it during the strike and resumed it. There are lots of legitimate reasons they would want to buy back shares - if they are using shares as payment for debt it keeps their interest rate lower.
Regardless, even with the buybacks, their share price is still lower than when the strikes began.
I shouldn’t have to be the one to tell you this, but the benefits unions fight for, like pensions and health plans, are largely stored on asset markets. Unions are not fighting to destroy the stock market.