They're using the aperture synthesis technique developed for radio telescopy in the early 50s. I haven't read up on it for yonks, but I'd guess that a telescope in LEO wouldn't make that much difference, since it would only increase the baseline by (scope altitude)/(Earth diameter). A telescope on the moon would be more effective but out of eyeshot for 2 weeks every lunar month.
There's the problem of data-collation, too. The EH network gathered too much for electronic transmission: they had to store it on stacks of CDs/DVDs and send it for processing in crates.