If it were anyone other than Trump, citing “unnamed US officials” wouldn’t be all that convincing.
But Trump’s positions and actions make this scenario plausible. Especially a scenario that begs for an explanation. Why would Russia intervene in a US election, and why would Trump willingly be their puppet (and both of these conditions are pretty obvious)?
Either Trump is being bribed (with some YUUUUGE bigly deal) or he’s being blackmailed. The idea that he is just fangirling over Putin on his own doesn’t make sense.
Still, I would feel more comfortable if we had more to go on than “unnamed US officials”. But even that merits some sort of investigation. And if this is true, it’s not going to stay a secret forever.