I don't see why they should think so given the Germans haven't shared anything of substance and are merely theorising as to what happened to Madeleine McCann.
Do you honestly think that SY would like to see the Germans solve this case when the Portuguese and the English have failed to do so?
I don't think we can discount that perhaps SY and the McCanns know quite a bit more about the BKA evidence than what has been let on publicly. And that perhaps the reason all parties (including the BKA) are intimating otherwise is for other sensitive reasons. One of which might be that the BKA haven't afforded the PJ same level of knowledge about their evidence. HCW has made no secret that the level of cooperation with the PJ is not great and I think the reasons for that are pretty obvious. The PJ don't really want the BKA to solve this case and prove that Madeleine was abducted IMO. Whereas I think SY would feel vindicated that they were correct about her being taken and also that they played a part in bringing CB to justice.
Personally, I think the McCanns may have been privately told certain details of why the BKA so strongly believe CB is responsible via SY, although probably not all of the details about the evidence that supports it.
IMO people read too much into the SY's standard "still being treated as a missing persons enquiry" line. It doesn't necessarily mean that they don't believe CB murdered Madeleine. What else do we expect them to say? "The Germans are telling everyone that they haven't shared all their evidence with us, but as they seem pretty sure she was murdered, we're going to change our position to a murder enquiry now too"? Unlikely. Within the scope of a "missing person" enquiry, they will of course be investigating the possibility of murder, so the two positions are not contradictary. It's a very politically sensitive case, and any comments coming from SY will be very carefully worded. When you analyse them, to get any meaning from them you need to consider what they DON'T say, rather than what they DO.
The fact that SY, nor the McCanns, have made any real fuss about "apparently" being left in the dark, or shown any dissent, I get the feeling they have been told a lot more than what has been released publicly, even if it's not everything. The only contentious comment they seem to have made was about the "letter" that the BKA sent. When you read what they say though, the dig is not aimed at the BKA, it is aimed at the press. The McCanns were complaining about the accuracy of what the media had reported about the letter, which was that the letter supposedly told them there was proof that Madeleine was dead.
It seems to me that the press were using a mish-mash of their own interpretation around what HCW was saying publicly about the case and what he said about the letter. I think this is what the McCanns took exception to. After the McCanns made their statement, the Press then decided to make out that this directly contradicted what HCW had said, when in reality, it just contradicted what the Press had made out that HCW had said about the letter. In fact, some of the press made out the McCanns were claiming there was NO letter at all, which is not what they said either. The McCanns released another statement a few days later clarifying a letter WAS passed on to them via the Met.
This is what Wolters said about it after the McCanns made their first statement about the letter:
And today, Mr Wolters again insisted they had sent a letter to the McCann's saying in a statement: 'We have written to the parents.
'The Public Prosecutor's Office has never commented on the content of the letter and will not do so.' https://www.dailymail.co.uk/news/article-8426407/Kate-Gerry-McCann-hit-German-prosecutors-claims.html?ns_mchannel=rss&ns_campaign=1490&ito=1490