Eye charts use the same width (strokes and gaps), so originally it was just a starting point to get some kind of context for what 20/20 means.
For larger fonts I would agree, the assumption is not correct. Clearly the gaps are bigger than the strokes for a typical font. But as the font gets smaller (especially around 1.5mm) it's necessary to maintain a minimum line thickness, and as such the stroke and gap become similar.
Anyway, the final criteria is a gap of 0.29mm. As long as the font maintains this, it should be visible for 20/20 vision.
For larger fonts I would agree, the assumption is not correct. Clearly the gaps are bigger than the strokes for a typical font. But as the font gets smaller (especially around 1.5mm) it's necessary to maintain a minimum line thickness, and as such the stroke and gap become similar.
Anyway, the final criteria is a gap of 0.29mm. As long as the font maintains this, it should be visible for 20/20 vision.