The Isle of Portland is what Wikipedia defines as a "tied island". That is: an island that's joined to a larger land mass by "a spit of beach materials" (known as a tombolo).
The tombolo in this case is Chesil Beach, which meets the mainland at Abbotsbury, about five miles to the west. But Portland is also connected to the mainland by the road referred to in the question, which uses an incomplete spit of land (bridging the narrow gap in the spit).
Portland Bill is the southernmost tip of the Isle of Portland. It is indeed connected to Weymouth by road, although the road does traverse the entire island before reaching the spit of land referred to above, and is not exactly straight or particularly direct.
The Isle of Portland is often incorrectly referred to as Portland Bill. Not that I'm suggesting that this is necessarily the case in this question.
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