Not counterfeit, not stolen
Not a trademark violation (searched on DS18B20 trademark shows nothing, and no 'tm' on the chip next to its designation)
Not a copyright violation (the dies are different)
Not a patent violation (ancient chip).
When I search Aliexpress for "DS18B20 temperature probe" there are plenty, but none of them claim it is the Maxim chip and DS18B20 is as generic as NE555 is to timing chips, or AA is to batteries. Is an AA battery a counterfeit? I don't know who owns the trademark even.
As for this: "Among the sensor chips analyzed, Petrich speculates some were stolen from the Maxim production line, some are less accurate than they should be in their temperature calculations"
Seconds are seconds. Nobody is going to break into a factory and steal chips worth pennies, and happen to only steal the sub-par chips that just happen to be lower spec ones!
A more usual thing: they test them, the out of spec ones are sold as seconds, sold off to crappy cheap temperature probe makes who knock out $0.83 temperature probes. You then buy those probes and spend a lot of time checking they don't meet Maxim's spec and then writing a lot of words.