Ice shelves (up to a few hundred metres thick) do have buttressing effects but sea ice that is what 1- 3m thick, and often quite mobile, buttressing effect? Surely it is too thin and mobile to hold back glaciers? Landfast sea ice may be less mobile, but if 'sea ice' means it is all formed from sea water rather than some of the ice being allowed to be from land, does it get thick enough? sounds rather dubious to me but I am no expert.
I think the confusion here lies in the interpretation of the word "buttressing". It is probably quite common to think in terms of the flying buttresses used to great effect since the 12th Century. These were typically designed to provide an inward force on church walls which would counteract the outward pushing effect of the roof mass. As such, these buttresses were, by necessity, pretty substantial lumps of masonry.
Although sea ice does present something of a barrier to the egress of the ice in a glacier by means of mechanical inertia, that is probably only a second order effect. What sea ice really does is to help reduce basal erosion at the calving front of the glacier (or indeed, of an ice shelf).
See, for example, this NASA article...http://earthobservatory.nasa.gov/Features/SeaIce/
The salient part is just below the photo of a polar bear on some sea ice, wherein the article states that..."The sea ice layer also restricts wind and wave action near coastlines, lessening coastal erosion and protecting ice shelves."
Buttressing effects of a more mechanical nature are discussed here...https://nsidc.org/cryosphere/sotc/ice_sheets.html
I recall a discussion a year or 3 ago in relation to Petermann Glacier concerning sea ice inhibiting or not inhibiting the glacier's advance. ...
Possibly the scientist to which Tor refers was Jason Box. Please see...https://nsidc.org/cryosphere/icelights/2011/09/greenland%E2%80%99s-glaciers-and-arctic-climate
In that article, Jason states that..."... The decline in sea ice could speed up ice loss in Greenland. “It is reasonable to speculate that changes in sea ice duration and concentration in the vicinity of glacier fronts should impact their stability,” said Box. “As the sea ice melts, the ocean can be stirred up more by strong Arctic winds and change fjord water circulation and the sub-marine melt regime.” Winter sea ice also acts as a buttress against glacier ice flow, seasonally slowing the flow speed. An earlier break-up and later freeze-up of sea ice in the fjords may play a role in the ice sheets' mass balance ..."