Out of curiosity, what does the EXIF tagging show for the f-stop? From what I've seen when working with MF lenses, I'm fairly certain the lens can't lie to the camera about its wide-open aperture, or else every shot would be underexposed by a stop. But perhaps it could get around that by lying about every setting equally. If that were the case, wide-open shots would claim to be f/5.6 at the long end, even though they really can't be.
First of all, the EXIF data is always correct. But somehow third party lenses have had a workaround that bypassed the f/5.6 maximum aperture limitation for many years. I understand that the trickery is not so much about metering as it by bypassing that limitation. Magic Lantern software also can bypass that same limitation for all lenses, so it obviously more of a software limitation than it is a physical limitation.
After studying the lens protocol, I think I get it. The camera asks the lens to report its maximum aperture, but that's the maximum aperture for the whole lens, not for the current zoom setting. When the camera sends a command to fully open the lens, the lens reports the actual aperture. I'd imagine that the check to decide whether to autofocus or not is based on the maximum aperture reported by the lens, rather than the aperture that the lens reports when the camera tells it to open all the way at the beginning of focusing.
So as long as the lens says that it can open up to f/5.6 or wider, even if it really fails 100% of the time in practice, the camera will make the attempt.
With that said, I only spent about three minutes looking over the lens protocol, so I could be wrong.
Of course the lens 'fools' the camera to think it's f/5.6. This is similar to the 'tape trick' on 'reporting' teleconverters or using a 'non-reporting' teleconverter to AF on non- 'pro' bodies even at f/8.
5D3, 5D2, Sony α6000, G16 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 2.0x III, 70-300L | E-mount: SY12 f/2, 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses