A full strike is definitely more effective, but that doesn't mean that a slowdown strike does not have its place. It has been used historically by the same great grandparents that went on strike as well as more recently. It can be used on its own or as a precursor to a full strike
I don't think that we should discount the idea just because a full strike would be better. A slowdown strike could be organized next week, could expand as ppl learn about it, could be organized without waiting for people to form unions or set up mutual aid. The website you shared talks about earning trust in nonprofits for months to set up mutual aid networks. Setting up a union also takes time. People want to act now. A slowdown strike can also build worker unity