At a minimum, it's a bad comparison metric. It's like with any software rewrite: the first 80% is the easy part. It's when you get down to the fine details of handling specific edge cases that things get really complicated. (Seeing this in a rewrite I've been working on myself.)

Semi-similarly, there's three reasons why React (and other libs) have that many issues: they've been around for a while, they're very popular, and the wide variety of usages has led to a number of edge cases being found. Svelte, being new, hasn't run into any of those things yet. Things like style of development can also affect things - some communities might file more issues than others.

We need a robust testing system to reduce bugs, improve performance and do some recommendation where code quality can be simplify or better security. I believe not many community can affordable the time to write testing, even expertise skived and problem appear at compile time.