By 81 Porsches were heavily galvanised but they are now circa 20 years old & that galvanising has got abraded away over the years. Certainly 944 sill problems are becoming more common & it is a lengthy & terefore costly job to do properly. Not heard so much about 928s having sills done, maybe because the get more care & attention so less likely to rust, better built in the first place or maybe so thirsty that they don't do enough miles to rust
Personally I'd want to know who did them & possibly see some pics. If they are just welded over covers then it'll be being done again in the near future. Mine (944) were multiple layers of thin steel & masses of filler.