It's not an excuse, it's cold hard analytics.
Not every player has the luxury to just turn on a switch and run hard to finish the chances that they already are 100% certain will come for them. Whether or not Ronaldo can finish, he'll still have an incredible amount of clear chances per game to try. Messi has a much more difficult role to execute, he's not getting the ball point blank in front of goal with the simple expectation to finish.
Messi gets the ball in much more difficult positions than Ronaldo, has a far larger path to goal than Ronaldo with many more obstacles, and is expected to contribute in a lot more different ways. There isn't just an 'on' switch you can flip to focus and run harder and be more successful. If Messi had 'run harder' today I doubt much else would've happened, he would have to run back to collect the ball from Busi to ever see it.
There's a reason why a lot of players similar to Messi (Hazard, Neymar, Robben, etc.) when having poor games in the CL show many of the same shortcomings. It's because it takes a certain amount of favorable conditions for them to perform the way they do, otherwise they can't. Ronaldo on the other hand gets his most advantageous conditions every single game without fail, tell me the last time RM's midfielders and fullbacks struggled to create open chances for him in the CL?