Because it was reported as a proposal to to start back some 387s from Swindon (Morning only) , in any case, the intention has been to clear 387s through to Cardiff as Crowdbusters for Rugby and other events..
Obviously I'm not suggesting demand from Swindon to Cholsey, Goring, Pangbourne, Tilehurst!
Where this came from was interpretations of this line in a presentation given in 2015 by FGW/GWR about future service plans.
Additional Oxford to London and peak hour fast trains from Swindon (am) to/from Didcot (am & pm)
This appeared in a section headed 'London & Thames Valley', so an assumption was made that this was a reference to operation by FGW/GWR's LTV services group, which would be using emus (a mixed fleet of 387s and 365s was planned at the time).
Instead, what is going to happen from December is the provision of an IET service starting from Swindon at 06.45, and the first train of the day from Cheltenham, instead of being a dmu terminating at Swindon, will be an IET that leaves Swindon for Paddington at 05.56 - thus fulfilling the pledge of more morning peak trains to London.
387s could appear at some point in future, but I expect GWR want to see how the new services bed in and how passengers's habits adjust before making any decisions about other options to adopt, if necessary.