Which one? The first?
Most entry level texts don't drive home the relationship Ekinetic = (Mrela-Mrest)c squared. At least in my experience. The poster was unclear as to an underlying reason for the Mrela tending to infinity as v goes to c, at least that was my read. As you accelerate a mass, you add kinetic energy. Which manifests as additional inertial mass. Which requires an ever increasing amount of energy to accelerate the mass. The limit of this is M goes to infinity as v goes to c. If you're reasonably skilled with differential equations you can derive the equation for Mrela from the postulate that Ekinetic=(Mrela-Mrest)c squared, and nothing else.
As to the second, I believe I misunderstood the poster. Upon rereading, I think he's jumping reference frames and confusing himself.