The point is that if you test more, your rate will be much higher. Right now we have far more unreported cases than in the USA. With all things accounted for, the true covid rate in US is still a lot higher but not by the margin that the numbers indicate
This is not true at all. If you test more the number of cases you find may go up but that is not the same as the positivity rate going up. The positivity rate reflects the prevalence of the virus. The higher the rate the generally more prevalent the virus is in your population.
Ontario with a population that is just under 14.5M people tested between 20000 and 30000 people a day of late. Yesterday it was 21,425 or 1.48 test per 100K in the population
https://www.ontario.ca/page/how-ontario-is-responding-covid-19
The US is now testing testing roughly 700K which is 2.19 per 100K. So proportionally this is roughly 50% higher than Ontario per capita. Ontario had 121 positive tests today. If the same testing rate as the US was used with the same positivity rate Ontario might have expected 182 case. Taking population into account at this rate the US would have had around 4100 cases. They had 57549 or roughly 14 times what you would expect if the virus was equally as prominent in the US as it was in Ontario.
Daily State-by-State Testing Trends
Florida has 21.5M people. They tested 49,177 people with 9488 positive tests. That is a testing rate of 2.28 per 100K which is slightly higher than the US average. But if you look at the equivalent testing rate in Ontario extrapolate to the Florida population you should expect 277 cases if the virus was as prevalent in Florida as it is in Ontario. That means Florida's numbers are roughly 34 times what you would expect.
Edmonton tested 3644 people in a day most recently. That is a just under 4 tests per 100k. So the testing rate in Edmonton is significantly higher than in any of the above places. They had 21 cases. Convert those to Florida and you would expect 285 positive tests in Florida if the virus had similar prevalence.
Now none of this is exact because who gets tested varies across jurisdictions. But the claim that this is just a reflection of more testing is fundamentally false.