The core argument is that Marek kills in 10 days or less, which is too soon for viral spread to happen. MDV vaccines keep chickens from dying, thus spread of hot variants can occur. Since covid doesn't kill that much, Marek phenomenon can't happen. The obvious issue with the argument is that the hot lethal variants are observed post vaccine availability. Earlier strains were "mild" or "virulent", but not "very virulent plus". "Death was relatively rare".
From an 1998 article on Marek, there appear to have been at least 3 jumps in Marek virulence, one in the late '50s pre vaccines (mild => virulent) and two post vaccine availability (virulent => very virulent, very virulent => very virulent plus), see chart on page S50.
https://www.tandfonline.com/doi/pdf/10.1080/0307945980841929...
From Andrew Read's own 2015 article:
"MDV became increasingly virulent over the second half of the 20th century [19,21–24]. Until the 1950s, strains of MDV circulating on poultry farms caused a mildly paralytic disease, with lesions largely restricted to peripheral nervous tissue. Death was relatively rare. Today, hyperpathogenic strains are present worldwide. These strains induce lymphomas in a wide range of organs and mortality rates of up to 100% in unvaccinated birds."
https://journals.plos.org/plosbiology/article?id=10.1371/jou...
The conclusion of the rebuttal article you linked to is rather terrifying. If there ever was a "the road to [chicken] Hell is paved with good intentions", this is it.
"In the history of human and animal vaccines, there have not been many cases of vaccine-driven evolution. But in every one of them, individuals and populations have always been better off when vaccinated. At every point in the 50-year history of vaccination against Marek’s disease, an individual chicken exposed to the virus was healthier if it was vaccinated."