Thats correct which is why the lay side of the market currently adds up to 100% or slightly over on occasion. Punters are looking to back the field and then should a favourite not play due to coronavirus (or whatever) then the odds will shorten on the bets already placed and not cancelled and the punters will be up. Makes it very hard to provide liquidity pre the in play market.
For anyone who thinks this is not too important, Dustin Johnson pulled out of a recent event as the favourite when he tested positive. So its a real risk and has been costly to me a few times this season.