It kind of sounds like this is part of a paper that is detailing seemingly large amounts of predation from cats of which the majority is attributable to un-owned cats which I gather you reckon means "outdoor" owned cats aren't a big threat to wildlife populations since they aren't responsible for the greatest amount of the total predation from cats overall.

But, without the context, the numbers cited sound instinctively like 'big' numbers so if the total magnitude of predation from cats is large and "owned" cats are responsible only for a fraction of it, their contribution could well be substantial nonetheless. Not knowing the scope or the details of the quoted paper it's unclear if it goes in to what the estimated proportion is other than not the majority and its unclear how much predation can be tolerated by the populations upon which cats, both owned and unowned, prey.

For example maybe owned cats are responsible for 40% of the total predation by cats on local wildlife in an area with the remaining 60% being attributable to un-owned cats. This would make un-owned cats majority responsible for the predation yet you could reduce the total predation by 40% if owned cats were all kept indoors in that hypothetical. The actual numbers are likely different and could well be much more slanted between owned vs un-owned cats' share of predation but if the estimates for the sustainable amount of predation certain populations can withstand are below the current total amount of predation then removing even a smaller fraction might be the difference between endangerment and extinction.