It seems so, yes.
UCSC uses a 10Kb definition and representation for telomeres in human. In the table you shared this is consistent across all chromosomes, not only chr21. This 10kb representation is an approximation to the mean value observed in newborns and is supported by evidence like the one linked below.
Taken from Wikipedia:
A telomere is a region of repetitive nucleotide sequences at each end of a chromosome, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. For vertebrates, the sequence of nucleotides in telomeres is AGGGTT, with the complementary DNA strand being TCCCAA, with a single-stranded TTAGGG overhang. This sequence of TTAGGG is repeated approximately 2,500 times in humans. In humans, average telomere length declines from about 11 kilobases at birth[ to less than 4 kilobases in old age, with the average rate of decline being greater in men than in women.
2,500 x 6 (TCCCAA) = 15,000
Two interesting additional facts:
The minimum telomere length needed to ensure human telomere protective stability in white blood cells is 3.81 kb
Mouse has a 100Kb telomere representation in USCS