  HD 21749 is a bright ($V=8.1$ mag) K dwarf at 16 pc known to host an inner
terrestrial planet HD 21749c as well as an outer sub-Neptune HD 21749b, both
delivered by TESS. Follow-up spectroscopic observations measured the mass of HD
21749b to be $22.7\pm2.2\ M_{\oplus}$ with a density of $7.0^{+1.6}_{-1.3}$
g~cm$^{-3}$, making it one of the densest sub-Neptunes. However, the mass
measurement was suspected to be influenced by stellar rotation. Here we present
new high-cadence PFS RV data to disentangle the stellar activity signal from
the planetary signal. We find that HD 21749 has a similar rotational timescale
as the planet's orbital period, and the amplitude of the planetary orbital RV
signal is estimated to be similar to that of the stellar activity signal. We
perform Gaussian Process (GP) regression on the photometry and RVs from HARPS
and PFS to model the stellar activity signal. Our new models reveal that HD
21749b has a radius of $2.86\pm0.20\ R_{\oplus}$, an orbital period of
$35.6133\pm0.0005$ d with a mass of $M_{b}=20.0\pm2.7\ M_{\oplus}$ and a
density of $4.8^{+2.0}_{-1.4}$ g~cm$^{-3}$ on an eccentric orbit with
$e=0.16\pm0.06$, which is consistent with the most recent values published for
this system. HD 21749c has an orbital period of $7.7902\pm0.0006$ d, a radius
of $1.13\pm0.10\ R_{\oplus}$, and a 3$\sigma$ mass upper limit of $3.5\
M_{\oplus}$. Our Monte Carlo simulations confirm that without properly taking
stellar activity signals into account, the mass measurement of HD 21749b is
likely to arrive at a significantly underestimated error bar.
