I have not fought this out, so cannot say for sure how it plays out. That said, there is language in several regulations that suggests that members such as you should be retained. The only hesitation I would have in saying that I am confident that this would be the ultimate outcome is the "layers" of presumptions that apply to "fitness" decisions by the military and the deference that the courts give to these decisions (to briefly explain, once a board has met and determined fitness or unfitness, this is given a lot of deference by reviewing courts....it is easier to fight rating decisions; all of this is very complicated when you get down to the weeds so don't take this expedient explanation as sufficient to understand all of the intricacies).
A separate point or tactic that I mention (were I to go full bore trying to fight this issue) is that the regulations suggest that members be given an opportunity to transfer to another branch to finish their service. Were I to fight a case out on getting to 20 years of service, I would suggest that there might be some "hay" to be made by making the request....there are no DES specific provisions for transfers, so I could not say how the process should work...in the absence of that, I would just request a transfer to HRC (again, I am not confident of the efficacy of this, the value in this would be in documenting the request to use as evidence in later appeals...I think the request would likely be ignored or not acted on; however, I think it would be valuable if you wanted to appeal further- such appeal ending up in court in my opinion).