You are citing a paper that has been cited only six times in the peer reviewed scientific literature since it was published in 1984, which was almost 40 years ago. One of those six citations was a self-citation. Papers that are as resoundingly under-cited as that are not definitive.
With that, the "hydrostatic equilibrium" aspect of what makes a planet a "planet" simply is not well-defined. The cited paper definitely is not definitive. The bottom line from the cited paper should not be that Mercury and Venus are far from hydrostatic equilibrium. The bottom line one should deduce from that paper is that the metric used in that paper is not a good metric for hydrostatic equilibrium, and hence the low citation rate.
It is hard to find any paper that is definitively accepted as defining a good parameter regarding hydrostatic equilibrium. Mercury and Venus are very slow rotators and are close to the Sun, and hence subject to tidal forces. These get in the way of establishing a good metric. The Earth is still recovering from the glaciation that ended about 12000 years ago. Moreover, there are signs that parts of former tectonic plates have dived almost to the core mantle boundary. The Earth is not in hydrostatic equilibrium. The Moon and Mars also are not in hydrostatic equilibrium. There are fast rotators such as Haumea that are triaxial in shape. This makes little sense from a naive hydrostatic equilibrium point of view. As an aside, Mike Brown, the discoverer of Haumea, was one of the key killers of Pluto as a planet. Mike Brown proudly uses @plutokiller as his Twitter username. "Hydrostatic equilibrium" is not a good metric unless one uses "approximately in hydrostatic equilibrium" as a rather fuzzy qualifier.
Regarding the other two attributes:
Orbiting the Sun is well-defined, okay, but wow. That means there are eight planets in the entire universe. All of the exoplanets that have been discovered to date are not "planets." However, this part of the definition completely bypasses several potential problems:
- The brown dwarf / super-Jupiter problem. There's no clear dividing line between a brown dwarf and a super-Jupiter.
- The newly forming star system problem. Things that might eventually become planets are not quite yet planets in those newly forming star systems.
- The rogue planet problem. Whether planet-sized objects ejected from a star system still count as planets is debated, and that perhaps includes the hypothetical fifth giant planet that some posit was ejected from our solar system early in its formation.
The "clearing the neighboring" concept also is well-defined; there are multiple metrics that agree that the gap between the eight planets and the myriad non-planets is a huge multiple order of magnitude gap. We don't know whether this applies outside the solar system. It probably doesn't apply for newly formed star systems, but it probably does apply for star systems more than a few hundred million years old. Almost all of the exoplanets orbiting stars other than the Sun would most likely qualify as planets were it not for the "planets orbit the Sun" clause.
One of the chief proponents of the "clearing the neighborhood" qualification, Mike Brown (mentioned above) used as evidence for the proposed demotion of Pluto's status a previously written paper by one of the key opponents of the "clearing the neighborhood" qualification, Alan Stern, who is the chief scientist for the New Horizons spacecraft that flew by Pluto and is continuing to this day. Two other papers were also used, all showing a huge gap between Mars and Pluto.
That paper by Stern found a parameter with a vast six order magnitude gap between Mars and Pluto. In that paper, Stern proposed that the eight objects in the solar system that have "cleared their neighborhood" using his own parameter be called überplanets while the lesser objects that still appear to be round-ish be called unterplanets. The IAU decided to call them planets and dwarf planets, with the exception that moons did not qualify as either a planet or dwarf planet. Dwarf planets must be objects that orbit the Sun as opposed to orbiting a planet or dwarf planet. Stern's proposal would have designated some of the larger moons as unterplanets.