The biggest difference here is not related to the animals themselves, but the scale.

Much of beef's emissions has to do with land use changes and diet which are both a necessary (but unfortunate) part of managing 1.5 billion cows to serve as a primary protein source for billions of people. In comparison, there's somewhere on the order of 30 million kangaroos on the planet (2% of the number of cows) and I'd wager the overwhelming majority of them are wild, not farmed for meat.

The difference in footprints here shows the differences in management practices and the downside of commercial ranching. If everyone on The planet switched their 0.5 servings of beef per day for 0.5 servings of kangaroos, nothing would be fundamentally different in the environmental outcomes. We'd still be clearing forests in the Amazon, just now it would be for kangaroos.

Sustainable meat consumption is only achieved through dramatic reductions in consumption. People don't have to quit meat, but it does have to become a thing reserved only for special occasions. Like it or not, the only path to sustainable food consumption requires everyone eating veggies (including the dreaded tofu) most of the time. Getting beef fed things like seaweed increases the portion of yearly meals that can include red meat sustainably, but does not somehow eliminate the fact that there majority of people's meals need to avoid red meat (sorry folks).