The problem is that this lesson just makes the general statement that adjectives that end in -s, double the s and add e for the feminine, whereas the accompanying video states that most adjectives ending in -s, follow the standard rules except for those listed by OP, which take -sse ending, and 2 others that absous, dissous - which both drop -s and take -te, and tiers which drops -s and takes -ce. There may be a problem in the video description of those that are regular (ambiguous I think) but neither does this lesson note that there are exceptions to the -sse structure.
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