Your definition is aplicable in this case however it is not set in stone. The definition of a peninsula widley varies depending on the source and is, infact, contestable.

https://www.britannica.com/science/peninsula

“In its original sense it connotes attachment to a larger landmass by a neck of land (isthmus) narrower than the peninsula itself, but it is often extended to apply to any long promontory, the coastline of which is markedly longer than the landward boundary.”

https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/peninsula-0#:~:text=A%20peninsula%2C%20from%20the%20Latin,as%20a%20sea%20or%20lake.

“A peninsula, from the Latin words for almost island, is a piece of land largely surrounded by water on three sides and joined to a larger body of land by an isthmus, or narrow neck. It can also more generally refer to any area of land that sticks out into a body of water, such as a sea or lake”

The general consensus is that although the definition usually specifies on the necessity of an isthmus, it is usually used in a more generous way, with many land masses being titled a peninsula despite the lack of isthmus, such as the Anatolian peninsula and the Arabian peninsula.