If you have a cheap meter with lousy specs calibrated, you have official confirmation that the meter is lousy.
What if you have a cheap meter with really good specs?
eg. https://lygte-info.dk/review/DMMAnengAN870%20UK.html
Currently, most even cheap multimeters have very good accuracy, be it Aneng or UNI-T... However, when it comes to the meaning of the term "multimeter calibration", we must first establish what we mean by it. Unfortunately, in many countries certain procedures have different names and some confusion can arise here.
Heh... I could even mention UNI-T multimeters, which when new are sometimes better than BM869 or other multimeters of this class (if we talk about accuracy)
For example, a relatively cheap multimeter calibration can be performed and can either consist of programming the calibration data into the device so that the multimeter reads correctly. We can also receive a printout on which the multimeter is compared with the reference equipment - then using, for example, the table we have, we can manually calculate and correct the indications. We can, for example, do something like this for private use. This is only a calibration and we do not get a certificate
There are also cases, most often in companies where we need a certificate (this is defined by the relevant standards ISO etc.) from an accredited laboratory - such a document has a certain legal force and most often such a procedure is more expensive than the so-called. "calibration".
Going back to those cheap multimeters, the device is accurate, and even if we pay and get a certificate for the device (which must be renewed periodically), there is still the question of how much we can trust the multimeter in the intervals between certifications. Unfortunately, the accuracy of multimeters is not everything and as I mentioned when the equipment is new, even the cheapest ones are in most cases very good.
The problem is, unfortunately, that here we can never be sure how long the multimeter will keep these good parameters, unfortunately in the case of cheap multimeters it does not look so good. In such cases, it is worth having not so much as some reference sources, but a better quality multimeter with which we can periodically check such cheaper equipment whether something is broken and does not show measurements very distant from reality.
This is an example from my experience. For example, one of the multimeters I use more often is the Brymen BM869S - bought over 5 years ago. The equipment is heavily exploited, it has had several falls to the ground from a greater height. I don't treat him like an egg either. In fact, although the housing does not look so nice anymore, the multimeter still maintains the declared parameters, and the multimeter did not require calibration (saving calibration data in the eeprom memory).
Unfortunately, it is not so beautiful with cheaper multimeters, e.g. wear of a rotary switch or poor quality sockets (cheap metal plates) is already starting to affect the measurements, and their accuracy has deteriorated significantly. Currently, they are more suitable as an "indicator" to check whether there is voltage or not

Here in all this the most important thing is the awareness of all this, not whether we have this type of "calibrator" or even a certificate.
Personally, I prefer to use 6 1/2 digital meters to test the cheaper millimeters - this is where I feel more confident about the procedure. Why ? It's unlikely that all three of my lab multimeters will suddenly fail or lose their performance. So even if one breaks, I'll be able to spot it very quickly.
Their accuracy is just as more than enough to be used as standards for cheap multimeters, I don't need an 8 1/2 multimeter to test multimeters 4 1/2 digits and below.
In fact, this is a more reliable method of evaluating multimeters than certifying a cheap multimeter or having a single reference voltage source (which can also fail).
Of course I have some Chinese reference voltage source. However, it is not used as a reference source, but only as a time-stable voltage source - which is also sometimes useful in testing. Knowing the exact voltage of this source is not the most important thing here.