Correct; for example, there are many AR500 plates that will stop M855 just fine, but will be defeated by M193 at close range and/or out of longer barrels (the velocity of the M193 allows it to punch through the alloy plates). However, as I had previously noted, the ATP3 is ceramic and UHMWPE hybrid plate; traditionally, UHMWPE has struggled against M855, but could defeat M193. Thus, while the ATP3 was not tested against M193, I would cautiously say that it would probably protect against M193. I'll also note that this is how unethical folks sell their armor, by proclaiming NIJ Level III status, and glossing over the fact that just because the plate can defeat M80, does not mean that it provides complete protection against the various 5.56×45mm threats.
I'll note a similar issue exists with the STOP-BZs (which are nothing more than rebranded TenCate 6400SA); it was not tested against 7.62×51mm NATO M80, so while I'd cautiously guess that it could probably defeat a couple rounds of M80, it remains a guess at best. To further muddy the waters, the Velocity Systems API-BZ Standalone plates are rated for M80 multi-hit, and are actually rebranded, modified variant of the TenCate 6400SA.
Note that if you're not worried about .308, the STOP plates are just a little more than the ATP3, are tested against M193, M855, and 7.62×39mm MSC, and are a fair bit thinner, at about the same weight.
I'd reach out to Mike and bounce any concerns you have off of him, just for peace of mind.
I'd also check up on First-Spear to see how much give their plate panels have, in case you're getting a 10"×12" plate; it'd also be good to make sure that the thickness of the ATP3 won't pose an issue, even if you're getting a SAPI sized one. Just due diligence stuff.
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