Wasn't the U.S. being behind the bombing a leading alternative hypothesis when the news broke?
Even extending to the international community (which is generally a reflection of American politics, much to the chagrin of Europeans who are unwilling to admit it), there is mixed levels of enthusiasm for getting to the truth. The countries are all anti-Russia, and so the pipeline sabotage is generally seen as a "good thing" except by the countries with direct profit motive for it. However, those countries aren't about to publicly accuse US intelligence of carrying out the operation, because their relationship with US intelligence is too important to lose, especially given all the weapons and intelligence they provide.
It just seemed inexplicable to me at the time because of Biden's prior remarks. In that light I can't see how anyone wouldn't immediately assume the US didn't do it-- the US hadn't even denied it at the time!