What’s the deal? The font-size declaration in the style attribute was invalid, and Chromium allowed it, in direct contravention of both the SVG 1.1 and 2 specs, while Firefox disallowed it.
SVG 1.1 extends CSS 2’s font-size value to allow a unitless number on a font-size SVG attribute <https://www.w3.org/TR/SVG11/text.html#FontSizeProperty>, but when you specify it as a CSS style it must follow the CSS declaration form, which means that non-zero lengths must include a unit—<https://www.w3.org/TR/SVG11/styling.html#StyleAttribute> even explicitly mentions font-size as its example on this point.
SVG 2 removes the unitless number extension and defers to CSS Fonts Module Level 3 entirely for font-size, so that unitless non-zero numbers aren’t permitted even in a font-size attribute.
All up, it’s a bad example that doesn’t support the thesis in the slightest, because a rookie error was made. (There’s still something in the fact that such an error could be made so easily, but it’s not that big a deal, certainly not enough to suggest that text is unreliable beyond the fact that you don’t know what fonts are available.)
Text in SVG is just as good as text in HTML, except inasmuch as nothing implements runtime (font-dependent) line wrapping, so it’s kinda more like HTML with white-space:nowrap.
The interactions between presentation attributes and style properties are a bit fiddly and sometimes unclear. Take something like <rect x="2" y="2" width="calc(100% - 4px)" height="calc(100% - 4px)"/>; it’s not quite clear to me whether this is valid in SVG 1.1, though I think it should be in SVG 2. It works in both Firefox and Chromium, though Firefox logs a claim that it’s invalid in the dev tools. <rect style="x:2px;y:2px;width:calc(100% - 4px);height:calc(100% - 4px)"/>, on the other hand, is definitely fine.