Threads directly formed in hardwood can be surprisingly strong and last surprisingly well1, either if they're cut/tapped or 'forced in' by drilling an appropriate size of hole2 and literally screwing the screw or bolt in by force.
Opinions are divided about which method is stronger, and I suspect it actually depends on the wood species (fine-grained or coarse-grained) more than there being one best method. But in either case you can reinforce the threading if desired by dribbling in some glue. Superglue/CA is commonly employed for this today, but epoxy would work well too (likely better). If you want to reinsert the screw or bolt into the hole before the glue has set be sure to thoroughly grease or wax it beforehand!
So given how often table legs might need to be removed I'm sure you can get away without the threaded inserts. But use them if you want the peace of mind.
I'm unsure if these would be suitable for hardwood as most reviews I see mention "for softwood"
There are different types of threaded insert and even if using one manufactured specifically for use in softwood this is in relation to screwing them directly into the wood at the stated pilot hole size, i.e. used 'dry' in a pilot hole sized for softwood.
But note that for hardwoods the pilot should ideally be a different diameter than for softwoods, just as for any screw going into wood. Additionally any threaded insert can be epoxied in place for added strength.
If using epoxy, before inserting for the last time thoroughly degrease the outside of the insert with alcohol, acetone or lacquer thinner. See more on epoxying inserts in place here on Wood Magazine's site, Tips for using threaded inserts.
You may also find this previous Question of help: How can I ensure my threaded inserts go in straight?
1 This is even when used for tools and jigs, where the screw would be withdrawn and tightened many times more frequently than when used for attaching legs to a table.
2 Ideally not exactly the same size as if you were tapping. But many of us don't have a huge range of incrementally sized bits to pick from so it's often the case that you have to use what you've got. Tip: where bits in 0.5mm increments are not easily found, or are too expensive, it can be useful to get a set of Imperial bits as most are in between millimetre sizes.