The ABIM research pathway is not competitive per se. It is an alternate way of getting your internal medicine license in which you skip your R3 year, go to fellowship, and then do an extra year of research as a fellow. Then you sit for your internal medicine and subspecialty boards around the same time. Notice a few things that are not part of the ABIM pathway:
1) Guaranteed fellowship admission at your residency institution
2) You have to do residency and fellowship at the same institution
3) Funding for research during your fellowship years (e.g. a T32 training grant)
4) Your fellowship research is after your fellowship clinical training
5) Financial bonuses attached to being in the pathway
6) You apply during your M4 year
7) It's a separate program you apply to and rank as part of the ERAS/NRMP
Lots of programs advertise that they support the ABIM pathway. Some call it the PSTP. Some call it the "research track." Some call it the ABIM pathway. They all conform to the guidelines set forth by the ABIM but vary dramatically in the above. Some programs don't guarantee you a fellowship spot. Some guarantee you spots only in certain fellowships. Some programs don't fund your research during fellowship. Some programs you apply as an R1 into their own program and get in at the program director's discretion. Some programs have a limited number of spots that you match into, while others you just show a "preference" and rank the categorical program. The list goes on.
So, in a long winded way, some of these programs are competitive and take strong applicants because they're either at a prestigious school or they have lots of extra perks. Others are not competitive at all because they are bare bones. In general, having an MD/PhD helps you get into these programs, but it's by no means a prerequisite, and it won't help you much if your clinical grades are not so hot.