In the "11.5. Text layout – Algorithm" [1], step 8.5.1.2.4, which reads:

Let offset be the value of the ‘textPath’ element's ‘startOffset’ attribute, adjusted due to any ‘pathLength’ attribute on the referenced element (if the referenced element is a ‘path’ element).

there's an additional restriction placed on when 'pathLength' applies ("if the referenced element is a ‘path’ element"). This restrictions seems to stem from the initial landing of the text layout algorithm. However (IIRC), since then all the basic shape elements gained 'pathLength' powers. Maybe this restriction (parenthesis) should be dropped?

[1] https://svgwg.org/svg2-draft/text.html#TextLayoutAlgorithm