The use of calcium to reduce foaming makes sense in spas (waterbear has recommended 120-150 ppm and that seems to work well in spas) where foaming can be more common due to the low water volumes so high bather load, but in a pool it's not normally a problem as Ben noted.

As for calcium and vinyl liners, this was discussed in a thread at TFP where I wrote this post that notes that acid, not low calcium, can be a problem but mostly when higher calcium carbonate fill levels are used in the liner. Though it may be possible that vinyl liners with high calcium carbonate might need some CH in the water, that's not a certainty and as noted in the references such high calcium carbonate content has other negative properties.

So the original post by SunnyOptimism isn't just an opinion, but based on some information from other sources though extended and possibly misinterpreted. One should always reference original sources whenever possible, not only for proper attribution to them but also so people can find more details, determine proper context, and draw their own conclusions.