I think it points towards the likelihood that closing borders is one of the key steps to restricting Covid 19.
was one of the key steps.
The article is interested in the period around March, so yes 'was'.
I think you are right that there are assumptions that may be inaccurate.
I don't believe it fundamentally changes the general point that the virus entered the country from a number of different places over a period of a few weeks, which could be good info to inform future strategy.
This also would reinforce the point that having a travel hub (London, New York, Paris etc...) would give greater challenges in a pandemic such as this one.