This is a pretty bad recounting of history. Just from memory I can recall more of this and some missing details are important.

First you have to know that Intel licensed the instruction sets to AMD and Cyrix (and possibly others?) in the 1990s. If you were around at that time, you could buy Cyrix 486dx2/66, 486dx4/100, 486dx4/133 and other CPUs that were really first to operate at a multiple of clock speed. Earlier CPUs didn't do this. But these deals were two-way, meaning Intel had the right to use any x86 extensions other manufacturers created;

2. Intel didn't like this. They'd also lost a trademark dispute over 486 where USPTO said you couldn't trademark a number. This was entirely the reason the Pentium was called the Pentium and not the 586. Intel didn't want to share. The instruction set cross-licensing was another issue;

3. Because of this, Intel wanted to go 64 bit from scratch. You have to remember that at this time the whole CISC vs RISC debate was unsettled. There were a variety of RISC UNIX servers and workstations from companies like SGI, Sun, HP, DEC, etc. Intel wanted to compete in this space. So they partnered with HP and came up with EPIC as the architecture name. The first CPU was Merced and it was meant to be released in 1996 (IIRC) but it was years late;

4. Intel thought their market dominance could drive the market. Obviously this would leave AMD (Cyrix was out by this point) in the cold. So AMD came out with the x86_64 extensions for 64 bit support and Athlon was born;

5. Oh, additionally in the 90s we had the (initially) Megahertz but later Gigahertz race between Intel and AMD. This is because clock speed became a marketing point. It was stupid because it ignored IPC (instructions per clock) but consumers responded to it;

6. So Intel's moved from the Pentium 3 to the Netburst architecture of the Pentium 4, which was designed to hit high clock speeds. You have to remember that even in the late 90s a lot of people thought clock speeds would keep going up to 10GHz. Anyway, Intel "won" this Gigahertz race with the Pentium 4 but lost the war as I'll explain;

7. So in the early 2000s, Intel needed a solution for laptops. They came up with the Centrino platform. I think this was the first laptop where Wifi was a first-class citizen. Anyway, Centrino was wildly successful against any competitors, so much so that people tried to make desktops out of it but it was really hard to acquire the parts;

8. So AMD took the easy route and released the Athlon, which was widly successful and with Intel facing ever-longer delays on EPIC was in a bind. They were forced to respond. They adopted x86_64 and repurposed the Centrino platfrom to create the Core Duo and then Core 2 Duo chips for desktop. To this day, the heritage of the Intel Core CPUs can trace its lineage back to the Pentium 3;

9. AMD further complicated Intel's position by releasing server chips. This is what the Opteron was. And this became a huge problem for Intel. EPIC chips were wildly expensive and, even worse, it required basically a rewrite of all software from the OS level up, compilers included. For several years, Opteron really ate Intel's lunch with Opteron.

10. By 2010 or so Intel had cancelled EPIC and regained their group on server-grade chips (ie Xeons) and AMD's Athlon and Opteron had begun to fade. So Intel had basically won but, don't worry, the 10nm white whale was just over the horizon.

I guess my point is that the Athlon can't be viewed or judged in isolation without considering EPIC, Intel's cross-licensing deals, the Gigahertz race, x86_64 and the Pentium 3/4.

Athlon 64: How AMD turned the tables on Intel