Is Borealodon a stem mysticete? No.

Due to taxon exclusion
whale experts Shipps, Peredeo and Pyenson 2019 described Borealodon (Fig. 1) as a ‘new stem mysticete’. They falsely reported “The earliest mysticetes had teeth” under the myth of a monophyletic clade ‘Cetacea’. They omitted the actual stem mysticetes, the desmostylians (Fig. 2), from their cladogram (Fig. 3).

Borealodon has archaeocete teeth
(Fig. 1) because it is an archaeocete, basal to odontocetes. Some Borealodon teeth have a premolar-like shape with twin roots. Others have an incisor-like shape with a single root.

When more taxa are added,
as in the large reptile tree (LRT, 1876+ taxa; subset Fig. 2) Borealodon nests with Aetiocetus within the clade that includes tenrecs and odontocetes. Mysticetes (baleen whales) nest elsewhere in the LRT (Fig. 2), with hippos, mesonychids, desmostylians and anthracobunids (including Mammalodon and Janjucetus, which both have teeth).

The convergence between odontocetes and mysticetes
was documented in the LRT in 2016, but this has been completely overlooked by whale experts world-wide ever since, who continue to consider the clade Cetacea monophyletic several years after the LRT found otherwise (Fig. 2).

What is a stem mysticete?
A desmostylian is a stem mysticete in the LRT. Desmostylians have been traditionally omitted from mysticete studies by whale experts, including Biscont 2006, Boessenecker and Fordyce 2017 and Shipps, Peredo and Pyenson 2019.

References
Shipps BK, Peredo CM and Pyenson ND 2019. Borealodon osedax, a new stem mysticete (Mammalia, Cetacea) from the Oligocene of Washington State and its implications for fossil whale-fall communities. R. Soc. open sci. 6: 182168. http://dx.doi.org/10.1098/rsos.182168References
Bisconti M 2006. Titanocetus, a New Baleen Whale from the Middle Miocene of Northern Italy (Mammalia, Cetacea, Mysticeti) Journal of Vertebrate Paleontology Vol. 26, No. 2 (Jun. 12, 2006), pp. 344-354 (11 pages) Published By: Taylor & Francis, Ltd.
Boessenecker RW and Fordyce RE 2017. Cosmopolitanism and Miocene survival of Eomysticetidae (Cetacea: Mysticeti) revealed by new fossils from New Zealand. New Zealand Journal of Geology and Geophysics. 60(2):