Ladybird is 425k LOC. Chromium is 3.5m LOC.
Starting a new browser, using LLMs... Is not going to maintain enough context.
Whilst Andreas does use Copilot a fair bit [0], he tends to do a line at a time, frequently disagree and rewrites his own, before prompting again. That is... He basically uses it as a fancy autocomplete. Not much else.