Its not an "experimental" station so therefore not subject to the Speller amendment so I believe that the normal closure procedure should still apply. Only Scotrail/Transport Scotland can answer the question as to why as to why the formal closure procedure has not been invoked.
I think the point here is that there doesn't seem to be anyone at Scotrail or Transport Scotland who is proposing closure. All the quotes are merely about no more trains calling.
The Scotrail website says:
Trains will no longer call at this station from 9 December 2018. The closest alternative stations are Inverkip and Branchton.
The Register article says:
"Trains will no longer call at IBM Halt station from 9 December 2018. This has been agreed with Transport Scotland and key stakeholders and partners along the route. The closest alternative stations are Inverkip and Branchton. Our current timetable will remain largely unchanged and there will be time between the stations before and after IBM Halt that would allow trains to call again at the station should the IBM site be redeveloped."
So what has been agreed is that trains will no longer call at the station. The question to avoid the closure procedures being invoked is what is the appropriate taxi / bus etc replacement service offering? As the site access is privately owned it may be that the appropriate level of replacement service is zero. Key stakeholders and partners almost certainly includes the Local Authority and the land owner. it may well be that the landowner could force Scotrail / TS to provide a taxi service for their representatives should they wish.
I suspect the landowner is actual quite keen to ensure urban explorers, train enthusiasts and local neds are not coming down to the vacant site on the train to drink and set fires so are quite happy for calls to be suspended and do not require or desire any replacement service. Equally however the presence of a station will add greatly to the long term development value of the land so they will not want the station to be closed (and indeed they would almost certainly object to any closure proposal, as would the local authority). If I were the developer / Local Authority I would have requested from Scotrail / TS a commitment that the station will reopen at zero cost to me once the development on the site commences.
So it seems that mothballing may be in everyone's interests at present whereas closure would not be in anyone's interests. Therefore a closure proposal has not been pursued but everyone is happy for calls to be withdrawn at present.