The genomes of the Neanderthals and the Denisovans are
older than present human genomes,
therefore the assumptions that 
generations separate the
first haplotype and
the second haplotype from a common ancestor are wrong.
The Neanderthal bones from which the DNA was extracted were dated to
38,310 and 44,450 years before present day by carbon-14 accelerator mass
spectrometry (28).
This gives an average of 40 kya, which corresponds to
2,000 generations.
The corrected formula for the average IBD segment length
would be
cM.
The Denisovan finger bone from which the Denisovan DNA was sequenced is
also dated back about 40 kya.
Thus,
10 kbp correspond to 6,000 generations or
to 120 kya and 20 kbp to 3,500 generations
or 70 kya.
The correction described above adds 1,000 generations or 20 kya as compared to the
number if two present day genomes would have been compared, that is,
if human IBD would be considered.