It is their words (telegram) not ours, "keep it secret".
That is
your interpretation of their words.
Here are
"their words":17. October 26, 1940
Typed entry in file 4439-40 (10)
from Vaskess to Sir Harry
His Excellency
Submitted with 4 [Gallagher's telegram of October 17th] and 9
[Macpherson's comments of October 23rd] and with a draft telegram to
M. Gallagher for Y. E.'s [Your Excellency's] approval.
2. Perhaps a carefully worded letter should now be sent to the U.S.
Consul-General in Sydney asking him to obtain a description of the
sextant carried by Mrs. Putnam and any number or distinguishing mark
on it?
17a. October 26, 1940
Handwritten note in red in file 4439-40 (11)
from Sir Harry to Vaskess
Sec with telm appd -
2. yr Para. 2: better I think await the arrival of the remains etc.
Thinnest rumours which may in the end prove unfounded are liable to
be spread.
They didn't relent from that decision. England was already defending itself, and needed help from the US. I can't fathom possible motives either, but the possibility of coming clean was not done in summer, 1941 and after. Who had the bones last? As far as we know, it was Paddie MacDonald. MikeP
There was nothing to "come clean" about, even if they had decided the bones belonged to AE. There is no need to "fathom possible motives." The motivation is given in the text. To translate it into English, Sir Harry was saying, "We should raise the hopes of the Americans that we have found Amelia. It would be embarrassing to make them think we might have found her if, in fact, we have not."
Once Hoodless decided that
the bones found on Nikumaroro were from a male, all the excitement about the case dissipated. I don't remember a single syllable in the bones file about Noonan. The British acted as if AE was flying solo.