I wouldn't call it at all doubtful. The fact there's less cars on the road can only push excess mortality down, regardless of lack of exercise, so it seems sensible to attribute it all to the pandemic.

Also, suicides aren't significantly up,the physical and psychological effects of lockdown don't happen immediately, while a reduction of car deaths do.

e.g. UK suicides have not significantly changed in 2020:

https://www.ons.gov.uk/peoplepopulationandcommunity/birthsde...

So overall I think they're pretty safely assume that any excess deaths are coronavirus, especially as flu has all but disappeared too.