It came from p213 "As defined under Units (see Components, p. 20), in these rules the term infantry refers to any infantry unit, whether conventional or battle armor" combined with p216 "Damage done by one infantry unit to another always equals the standard damage inflicted; it cannot be reduced by using the Non-Infantry Weapon Damage Against Infantry Table". So by those rules, because p216 doesn't specifically differentiate between infantry types, then it applies to all infantry.
This has long been recognized as clashing with the p215 Attacks Against Conventional Infantry rules with the argument swinging back and forth. I prefer the p215 interpretation, which I think came up way back in one of the earlier BAotW articles (IS Standard, GD Standard, Sloth, somewhere around there), but recently I've been running into more and more players opting for the p216 interpretation as the correct one. It does have the advantage of avoiding peculiar circumstances such as battle armor equipped with a small laser and wielding an infantry MP Plasma Rifle in gloves able to do more damade with the latter than the former, for example, even though the laser is a more damaging weapon and both are energy weapons.
As is my wont when questions like these pop up, I did some digging in Rules Question threads and it looks as though Xotl recently declared on the p215 side, so the rules are presumably going to be cleared up in a future TW printing.