Borrowing is what the government ought to be doing in a recession in order to finance the spending used to boost aggregate demand. The question in 2020 was how long could the borrowing of such large amounts continue? There can be problems with a large public sector debt. An expansion of government borrowing might increase interest rates if investors require more compensation because they think the risk of holding government bonds has increased. This interest rate rise would affect the rest of the economy.
There is also a theoretical objection that government borrowing could 'crowd out' the private sector, especially when the economy is close to its productive capacity and unemployment of resources is low. This would not happen when there are unemployed resources in a recession.
The UK Finance Minister suggested that a high ratio of public sector debt to GDP was justified; it is the stability of the ratio that is important. This reasoning can be justified. Reducing government spending in order to reduce borrowing would risk stopping any recovery and make the situation worse. Economic recovery is necessary to restore economic activity, which in turn will restore the government's finances.
However, economists cannot know with any precision what the limits of public sector borrowing should be. For many years, Japan has maintained a ratio of public sector debt to GDP of over 200% with no difficulty. But what is suitable for Japan may not be suitable in a different type of economy.