That problem is known as relational division.
One solution is double negation. You can select all suppliers, for whom no product delivered by supplier X exists, that is not delivered by them:
select distinct other_supplier.SupID
from Inventory other_supplier
where not exists
(
select *
from Inventory supplier_X
where supplier_X.supId = 1 -- For supplier X
and not exists
(
select *
from Inventory other_product
where other_supplier.supId = other_product.Supid
and supplier_X.prodId = other_product.prodId
)
)