You're right on a few fronts. Seymour Hersch--who is a respected, Pulitzer Prize-winning journalist--has made these claims but has been extremely vague about where he is actually getting all of his information from.
That Daily Sun article you linked basically copies verbatim what Hersch posted on his blog site on Wednesday, April 14: that Ukraine President Volodimir Zelensky and many of his top generals have been misappropriating funds in excess of $400 million that were specifically earmarked for fuel purchases. Their major fault in embezzling these funds, as claimed by Hersch, has been that the diesel they've bought have come from Russian suppliers--essentially buying fuel from the enemy they're already fighting.
To back up his claims, Hersch references multiple times a "knowledgeable American intelligence official and "one estimate from analysts at the Central Intelligence Agency." You'd be right to call these sources extremely vague. Additionally, Hersch claims that the US confronted Zelensky in January about him "taking a larger share of the skim money than was going to the generals" and accusing him of corruption at the expense of the American taxpayer.
We can't really be sure if what Hersch is claiming is true. We are given very little and very vague information, yet Hersch's reputation precedes him; he is a seasoned journalist familiar with the verification process. As we've seen with the recently leaked Pentagon documents, much of American intelligence surrounding the Ukrainian war is top secret, so I wouldn't count on either the American or Ukrainian governments commenting on this potential fracture in their relationship any time soon, especially that both have worked hard to portray themselves as a single united front against Russia.
