Thermometers, let alone ones with 0.1°C precision, do not grow on trees.
Taking temp at the same part of your daily schedule, e.g., on awakening, will still show substantial variation based on just waking up earlier/later, or if something skews your sleep cycle -- and by substantial, I'm talking about >1.2°C.
Take temp at different times of day, and you're looking at >2°C normal variance between early morning low and mid-afternoon high.
Source: while training & competing at an international level, I took resting Temp/HR/BP readings 2x per day to regulate the training intensity schedule (unusual increases in resting values provided early warning for overtraining or illness, so could back off a smaller amount earlier rather than being forced to a larger rest later). What I found was that the temp numbers were extremely sensitive to the huge daily temp cycle, and that I had to constantly compensate reading that value.
It's really interesting, but unless you're very consistent and understand the daily cycle and variability, really hard to base anything on it (and I'm not even considering the F hormone cycle which is what they're apparently trying to measure).
Likewise, if I use sophisticated ML techniques to identify young students who have attention problems, but then the actual treatment is 1-on-1 tutoring (rather than, say, drugs), this is still a natural intervention. And it is reasonable to suspect that it has fewer side-effects on account of being natural, which must be traded off against it's lower effectiveness.
Personally, I generally find the distinction between natural and unnatural interventions mostly unhelpful (since there are plenty of natural ways to hurt yourself), but it's a meaningful distinction and one that can plausibly provide useful guidance to someone who is very risk adverse.
No, it specifically isn't abstinence. It's a combination of mostly non-abstinence with a window of abstinence timed with a thermometer.
A method can be called "natural" if it could successfully be practiced by cavemen. Selective abstinence based on counting days since menstruation could be called natural; cavemen could conceivably figure that out.
Needless to say, total abstinence is also natural.