What exactly is a healthy adult here?
I haven't been able to find a source that identifies what specific set of studies the FDA used to make this determination, and didn't immediately find a more detailed report the one linked here which is clearly meant for the general public as a public health statement rather than consisting of comprehensive professional-level guidance.
There are, however, various review papers that speak of the same 400 mg per day guideline. Because they are reviews of multiple studies, there is no single inclusion criteria for patients. I'll mention just a couple of them here.
Wikoff, D., Welsh, B. T., Henderson, R., Brorby, G. P., Britt, J., Myers, E., ... & Doepker, C. (2017). Systematic review of the potential adverse effects of caffeine consumption in healthy adults, pregnant women, adolescents, and children. Food and chemical toxicology, 109, 585-648. is a systematic review; as such, rather than having a specific "healthy" group they have an inclusion criteria for studies, including control groups from studies comparing to another group that may not be considered "healthy".:
“Healthy” subjects were defined as individuals who were not specifically described as having been hospitalized or diagnosed with disease and/or receiving medical treatment for a disease at the time of the study. As such, studies evaluating a healthy population (which included athletes, military personnel, and pregnant women, unless otherwise noted as unhealthy) were included. Studies in which healthy individuals were included as a control group (or similar) as part of a study on unhealthy populations (e.g., individuals with asthma) were included; however, only information from the healthy individuals was used in the assessment.
I'd say this is a fairly broad definition of "healthy".
Another study, Nawrot, P., Jordan, S., Eastwood, J., Rotstein, J., Hugenholtz, A., & Feeley, M. (2003). Effects of caffeine on human health. Food Additives & Contaminants, 20(1), 1-30., is not as systematic, but again "healthy" populations are defined in contrast to other, specific groups of particular interest. If a study is about arrhythmia, the "healthy" group is people without arrhythmia, if about hypertension, the "healthy" group is people without hypertension. For example:
Clinical studies have shown that single doses of caffeine<450 mg do not increase the frequency or severity of cardiac arrhythmia in healthy persons, patients with ischaemic heart disease or those with serious ventricular ectopia (Myers 1998)
(here, healthy means those without ischemic heart disease and without ventricular ectopia, a type of heart arrhythmia)
The effect of caffeine on blood pressure in habitual caffeine consumers and abstainers has been investigated in more than 50 acute and 19 repeated-dose clinical trials with healthy or hypertensive subjects(reviewed by Myers 1988, 1998, James 1991c, Green et al.1996). The results of the acute studies indicate that caffeine induces an increase in systolic (5–15mmHg) and/or diastolic (5–10 mmHg) blood pressure, most consistently at doses>250 mg/person, in adults of both sexes, irrespective of age, race, blood pressure status, or habitual caffeine intake.
(here, healthy means "not hypertensive")
More generally, then, I would read a recommendation about "healthy people" to basically be saying "we can't rule out that there are some specific populations that will have negative effects". They can only consider the scope of the studies available to review.
Each individual has a unique combination of health circumstances. If any individual wants advice about their personal caffeine consumption and whether they should use FDA's caffeine guidelines for "healthy" adults or some other guideline, they should ask their own physician. From the FDA's consumer guidelines linked in the OP:
Certain conditions tend to make people more sensitive to caffeine’s effects, as can some medications. In addition, if you’re pregnant, trying to become pregnant, or breastfeeding, or are concerned about another condition or medication, we recommend talking to your health care provider about whether you need to limit caffeine consumption.