In journalism, this is common lingo for “our journalists personally communicated with sources (more that one) that wish to remain anonymous.” It’s the guardian’s legwork and integrity for shielding anon sources here, not them copying it from some other site (which they’d link if they did).
To expand on Shrubbery, you can read the Guardian’s editorial standards and the base ones they follow: https://image.guardian.co.uk/sys-files/Guardian/documents/2011/08/08/EditorialGuidelinesAug2011.pdf
In journalism, this is common lingo for “our journalists personally communicated with sources (more that one) that wish to remain anonymous.” It’s the guardian’s legwork and integrity for shielding anon sources here, not them copying it from some other site (which they’d link if they did).