I don't give much for the odds of IG surviving, but they're so numerous and their weapons vastly outstrip those of even the Covenant themselves, it's difficult to think of the Covenant having much luck with an offensive. If Spartan IIs and IIIs are any indicator of how effective Space Marines would be, I think you're right. A single Chapter's Terminantor Company could more or less obliterate anything the Covenant threw at them, and seize or destroy any ship or craft they happened to teleport inside of an hour.
Factor in the the Covenant likely have no way to defend against the attacks of Sanctioned Psykers and Space Marine Librarians and they're really just completely outmatched in every category.
As a final testament to the sheer scale by which WH40k overwhelms the covenant, and to settle the matter of ship superiority once and for all, I post this comparison. A Covenant capital class ship is about the size of a small city and crewed by a few thousand Covenant. That's about the size of a small frigate or system defense ship in 40k terms. Imperial Capital Ships have crews upwards of half a million and are the size of small states. Any sector's battlefleet is comprised of at least a dozen or so of such craft, and their support ships.