that’s because a Trump win in Pennsylvania signifies that Biden is probably getting crushed among working class white voters, which probably means he’s going to struggle in Minnesota, Wisconsin, etc.
You can’t look at states in a vacuum, and so Nates prediction is looking at voting trends across the demographic and that’s why the whole “Trump wins PA swings the election for him” narrative is created.
In reality, Biden has a ton of other options that Hillary lost. Wisconsin, Minnesota, Michigan, Arizona, Georgia, North Carolina, Florida, Texas and Ohio are all in play and some even leaning Biden.