Well, kind of.
Bombardier isn't really a threat to Boeing (or Airbus) unless it starts making CS500s. Bombardier can duke it out with Embraer all day long and Boeing won't care.
Strategically, countering a possible CS500 is where the game is. So we now see Boeing's response with the MAX 7X idea being floated. This move if it happens could really disrupt future CS300 sales in order to prevent a CS500.
Or put it another way, a potential MAX 7X could be more troublesome to Bombardier vs. a potential CS500 is to Boeing.
It basically comes down to financial resources. Even assuming Bombardier wins Delta, but losing Republic and a good portion of the Russian orders, the CSeries order book is still pretty short, around ~ 250. They need to sell > 3000 just to recoup their investment back.
This model assumes zero competition from Boeing. If Boeing enters with MAX 7X then it's just that much harder for the CSeries to ever be profitable, and raising more debt money for a CS500 might be close to impossible.
Unless of course Ottawa and Quebec would agree to even more $$$ Billions.
Another wildcard is of course Embraer also crowding the market with a larger E2, but so far they're on the record that they don't want to compete in the 150-seat market.