The article you posted was from the WTM in October 2015, I think you can discount what the General Manager for UK & Ireland Jubi Prasetyo actually says as obviously the situation has changed since that article was written and I think Garuda actually made a press release in November regarding the transfer of their Jakarta route from LGW to LHR commencing at the end of March 2016 at the beginning of the IATA summer season and have supposedly gained a few evening slots with departures from Terminal 4 sharing with other Skyteam members and connections.
I have googled several times but can not find any schedule for LHR so it may be the fact that Garuda were not able to obtain the appropriate slots for their planned schedule from LHR or have decided to stay at LGW at least until they can find suitable slots at LHR.
I personally would like to see Garuda remain at LGW but looking at it from the airlines perspective it makes far more sense for them to transfer to LHR and move in with fellow Skyteam members at Terminal 4.