for example comparing the level of infrastructure development in China and the US reveals clear strengths of socialism over capitalism.
That's not whataboutism. That's just a comparison as you pointed out. Whataboutism is when you address a critique of your position by saying, "we're not the only ones though"
I can't think of any strawman arguments I've seen recently from leftists
This post is a strawman. It assumes criticisms of China are centred around infrastructure as opposed to other things. Unless OP specifically made this post in response to someone they had (or are having) a discussion with, I see no reason to generalize this as a position all "liberals" take.