The generic congressional ballot is tighter than the 2018 results yet many people are trying to claim that a large number of Republican held seats are up for grabs. I do not believe it.
I don't think anyone is saying that specifically - most people are pointing out though that there a good amount of R seats that are in districts that are quickly trending left, like this one, that are up for grabs. Trump/Rs have a particularly weakness in 2020 with the suburbs like this one, and there are a good amount of districts like this are slipping away from them.
Not to mention, the 538 average is D+6.1, which is lower than the final avg of D+8.7 in 2018, but there haven't been as many polls (esp high quality) lately, so given the MoE, it's not that far-fetched.