Yes, Google probably can achieve quantum supremacy by slapping a few more qubits. But a few more qubits were, in fact, not slapped yet. So quantum supremacy is very much not achieved, and Google should not claim so.
That's why arguing over whether it's really quantum supremacy is just not useful. People can always move the goalposts, so we might as well ban that word from discussion. The underlying fact, which nobody is debating, is that quantum computers are regularly doing tasks that are harder and harder for classical computers to.
And yes, the ECT is obviously false -- or at the very least, not obviously true. I'm surprised at how many CS people think it is some unassailable principle. Most CS courses devote precisely zero time to non-classical models of computation. If you've never studied a model of computation that could challenge ECT, then how can you be so sure of it? This is like being absolutely certain that I am the tallest person in the world, and refusing to ever leave my house to check.
The Extended version goes on to say a probabilistic TM can efficiently simulate all realistic models of computation. Quantum computers very likely violate the efficiency claim and this supremacy result is strong evidence in support.
Quantum supremacy is the potential ability of quantum computing devices to solve problems that classical computers practically cannot. The weaker term Quantum advantage refers to the potential to solve problems faster. In computational-complexity-theoretic terms, this generally means providing a superpolynomial speedup over the best known or possible classical algorithm. The term was originally popularized by John Preskill but the concept of a quantum computational advantage, specifically for simulating quantum systems, dates back to Yuri Manin's and Richard Feynman's proposals of quantum computing.
IBM's algorithm scales approximately linearly in the number of qubits. So, you'd need more than a few more qubits...
The reality is that quantum computing technology has been improving at the same steady pace for decades, and quantum supremacy was recently invented as a concrete but not very useful goal that people could realistically aim for in the near term. That is a totally standard and legitimate way of guiding technological progress.
The problem is that quantum supremacy is vaguely defined (what does "infeasible on any classical computer" mean? what computers can you use, and for how long?) but it is presented to the public as a sharp boundary -- which means that every company in the field has a huge incentive to claim they are the first. This IBM claim is not a fundamental objection. Given that everything they say is correct, Google could still achieve quantum supremacy by their standards just by incremental improvement, slapping a few more qubits on. So why even bother disputing Google's claim? Because it will set them back a few years, and that's enough time for IBM to make a claim of its own. That's why academics, who have no stake in what company gets the hype, will just consider this whole episode rather distasteful.