Ahh the old days of LAMP when the M was mysql and foreign keys were just a dream...
When you reduce complexity and take off the safeguards things get faster! Cock that foot gun and hope that it doesn't go off!
Can you do what the author suggests. You sure can and we did it for a long time with MYSQL. Should you? It depends on your team, how in tune they are with working with databases, sql etc...
There was a funny NoSql video that said something to the effect of, "if you are willing to sacrifice everything for speed, just pipe all your data to /dev/null."
Found it: https://www.youtube.com/watch?v=b2F-DItXtZs
If you are willing to forego 3rd normal form and just duplicate data like mad, and have a million column like a giant excel sheet, then you really don’t need foreign keys.
MyISAM I think was the table type that did NOT support FK constraints and was blazing fast. InnoDB had FK constraints.
You would fully normalize, use proper joins etc. But without FK constraints you could end up with orphan data... Not the worst thing in the world, depending on how the joins in your system were structured.
Care and diligence were the order of the day. You had to know your schema and make sure you were doing the RIGHT thing at all points in the stack.
If you duplicate data like mad, how do you ensure that the duplicates are in sync? If you avoid UPDATEs I could maybe see it working.