Yes, the Betting Exchange is for Member vs Member Bets eg, One (Bet Owner) vs One (unknown member), not One vs Many. An Open Bet can only be claimed by one member (first come, first serve).
Nothing prevents you from creating multiple open bets for the same thing tho, you can create the same Bet (Raiders score 20+ Points in the 4th quarter $1000) multiple times (in case you want to offer that same bet 5 times). I do this all the time (create a bet against one of my buddies and then offer that same bet to others by creating that same bet as an open bet multiple times. Sometimes I get multiple people to "bite", other times I either end up having them expire or a remove them and post different bets.