Upper/lower bounds define the growth rate in a particular situation (such as worst or best case).

So the statement is badly worded. It is true that the lower bound in all cases is \Omega(1) (it always requires at least one unit of time). But the reason is certainly not because there happens to be a situation where the algorithm is cheap to run. In other situations it is not so cheap.

Proper wording: The lower bound for the cost of sequential search in the best case is \Omega(1).