I think that page oversimplifies the rule for constructing comparative forms. One-syllable adjectives definitely take suffixes and three-syllable adjectives take words, but two syllable adjectives are difficult. I think this page is largely correct. For two syllable adjectives, some terminal syllables (-y, -le) require suffixes and some (-ing, -ed, -ful, -less) require words. The rest are OK either way (quieter, more quiet).