That's a really awesome news to hear because I believe that the step will cause a good bullish reaction in Bitcoin and it might be the first step towards the next bull run. I think the deal is beneficial for Bitcoin because it's currently not pumping the way we all expected it to pump. However, things can be different with this purchasing.
When the Tether buys that much worth of Bitcoin the pump is almost a guaranteed thing and investors might begin adopting Bitcoin once again after the bear run. The things will be really good in Bitcoin-sphere and it might help other altcoins to pump as well.
I believe that the deal will be beneficial for Tether as well because they will have more Bitcoin in their reserve thus investors won't fear about its failure as the other stable coins have gone through. The UST de-pegging has taken away interest of many investors from stable coins, as all of them now think that stable coins can be de-pegged and as a result they will lose their investment. This step can be a useful step to remove that fear from the hearts of those investors.