Well, Mitochondrial DNA CAN be linear (in some organism), see the wikipedia page on this
In most multicellular organisms, the mtDNA - or mitogenome - is
organized as a circular, covalently closed, double-stranded DNA. But
in many unicellular (e.g. the ciliate Tetrahymena or the green alga
Chlamydomonas reinhardtii) and in rare cases also in multicellular
organisms (e.g. in some species of Cnidaria) the mtDNA is found as
linearly organized DNA. Most of these linear mtDNAs possess telomerase
independent telomeres (i.e. the ends of the linear DNA) with different
modes of replication, which have made them interesting objects of
research, as many of these unicellular organisms with linear mtDNA are
known pathogens.[19]
For human mitochondrial DNA (and probably for that of metazoans in
general), 100-10,000 separate copies of mtDNA are usually present per
cell (egg and sperm cells are exceptions). In mammals, each
double-stranded circular mtDNA molecule consists of 15,000-17,000[20]
base pairs. The two strands of mtDNA are differentiated by their
nucleotide content, with a guanine-rich strand referred to as the
heavy strand (or H-strand) and a cytosine-rich strand referred to as
the light strand (or L-strand). The heavy strand encodes 28 genes, and
the light strand encodes 9 genes for a total of 37 genes. Of the 37
genes, 13 are for proteins (polypeptides), 22 are for transfer RNA
(tRNA) and two are for the small and large subunits of ribosomal RNA
(rRNA). This pattern is also seen among most metazoans, although in
some cases one or more of the 37 genes is absent and the mtDNA size
range is greater. Even greater variation in mtDNA gene content and
size exists among fungi and plants, although there appears to be a
core subset of genes that are present in all eukaryotes (except for
the few that have no mitochondria at all). Some plant species have
enormous mtDNAs (as many as 2,500,000 base pairs per mtDNA molecule)
but, surprisingly, even those huge mtDNAs contain the same number and
kinds of genes as related plants with much smaller mtDNAs.[21]
https://en.wikipedia.org/wiki/Mitochondrial_DNA
In the case of your sequence, it only means that it has been entered in NCBI as a linear piece of DNA (it is usually the case when you sequence only a piece of a circular DNA for example)
It does not mean that within the organism it is linear !
I hope this answers your question !
