The point is whether 'And all the air a solemn stillness holds' should be read:
(a) 'And a solemn stillness holds all the air.' (subject = a solemn stillness)
(b) 'And all the air holds a solemn stillness.' (subject = all the air)
I think bakaKanadajin is saying that for (b) to work, there should've been a proposition like 'in' before 'all the air':
'And in all the air a solemn stillness holds.'
I think that for (b) to work, there should've been a comma after 'air':
'And all the air, a solemn stillness holds.'