You all keep saying this. In the last six midterm elections, three have had the non-Presidential party gain, two have had the Presidential party gain, and one has had no change in overall composition.
It seems like the House is much more likely to dramatically swing against the President's party in a midterm.
The numbers for the House during the last six is still 4-2, nothing that completely off-sets, and even within those two, one was second term and one was first term. If Biden has a positive midterm, those numbers become 3-3-1 and 4-3, respectively, which shows that anything can happen.