This hinges as bit on what "as easy as" means. We clearly can't go to Mars today because we don't have the technology. We do have all the bits and pieces in theory, but we haven't build anything that can actually do it. SpaceX is famously trying to do exactly that with Starship/BFR.
The discussed paper is Low-thrust trajectories for human missions to Ceres. Frank E.Laipert, James M.Longuski (Published in Acta Astronautica Volume 95, February–March 2014, Pages 124-132). We can find in the conclusion:
We can draw the following conclusions from this mission design study.
A human mission to Ceres could be made feasible with the appropriate investment in propulsion and in-space power technology. Given the assumptions used here, the total IMLEO, with a 10% margin, would be 458 Mg. The mission architecture presented here would deliver a total of 155 Mg of payload to Ceres over two missions. Four heavy-lift launch vehicles would suffice to carry out such a mission.
Nuclear electric propulsion technology enables human exploration at Ceres because it has a relatively low specific mass (i.e. about 5 kg/kW) and it avoids a costly impulsive capture maneuver at Ceres. Electric propulsion technologies capable of processing input power up to 11.7 MW (or more) should be further developed to open the possibility of exploring Ceres.
Total mission times of less than 2 years (for the crew) are possible with nuclear electric propulsion. In the absence of a proven method of blocking deep-space radiation, limiting mission times is the best way to limit the danger to the crew.
So the point is, its easy if we have that power source.
Still, the investigators said that even without these options, their work demonstrated that a human mission to the dwarf planet was feasible.
Hm, what I found is:
4.1 Key technologies to develop
The primary technology enabling a human mission to Ceres is a nuclear power system capable of generating 11.7 MW of power with a specific mass of 5 kg/kW. A mission may be feasible with a smaller power system of around 8–9 MW, however more propellant and a higher IMLEO would be required. In addition to the power system itself, an electric propulsion technology capable of converting that power into thrust with an efficiency of about 70% is needed. (highlight mine)
That doesn't seem to confirm this statement.
That being said, I don't think the statement "We can go to Ceres as easily as Mars" is too misleading. Delta-v to intercept mars is about ~4300 m/s from LEO, for Ceres its more like ~4500m/s (doing some back-of-the-envelope math), that is not to far off. You can not aerobrake on Ceres though, which makes landing a lot harder, but launching is also easier. The problems faced are certainly different.