It doesn't index text in local image files, and it doesn't index over the full text of all the webpages and epubs I've read.
I believe they meant they want something that searches only content the user has personally directly accessed, not ALL local content.
Ordering by "Date Last Opened" or "Date Modified" does a fairly good job in some cases
This does introduce sorting contention though, what to sort for first? relevance or date accessed? Ideally you would want to introduce date accessed as an aspect of relevance itself.
Spotlight on Mac will show you recently opened, or frequently opened, files first.
I thought we all disabled access time (mounting with noatime) to avoid trashing SSDs so quickly.