So probably everyone has seen the latest chapter. Here we see Minato using Kyuubi chakra.
I mean to state that Minato has only KM (also called KCM) and not BM. All you have to look at are the specific markings that are characteristic to KM and BM
Here we can see Naruto's KM:
And here are panels with Naruto's BM
And even tough Minato has a bit longer stripes on hands and legs, but the main part - the chest and the stomach is the same as Naruto's KM and is very diffrent from his Bijuu Mode.
People started to think that Minato has BM because he already wore cloak which normal KM doesn't create unlike Bijuu Mode.
Think about it logically. Minato has half of Kyuubi's chakra without the mind of the bijuu. It is the same as if Naruto had only this chakra which is separated from Kyuubi:
Minato can't really have BM because it is something that is created from the link between the host and the bijuu and then melding their chakra together:
During BM their minds are also linked so there is no way for Minato to be able to get into BM.
Minato has KM and not BM because of the wrong markings and lack of mind of the bijuu inside of him that is needed for BM