Can someone please confirm for me that a matching contribution must be 100% vested in order to borrow it for purposes of the ADP test?

That is correct. From §1.401(k)-6:

Qualified matching contributions (QMACs). Qualified matching contributions or QMACs means matching contributions that, except as provided otherwise in §1.401(k)–1(c) and (d), satisfy the requirements of §1.401(k)–1(c) and (d) as though the contributions were elective contributions, without regard to whether the contributions are actually taken into account under the ADP test under §1.401(k)–2(a)(6) or the ACP test under §1.401(m)–2(a)(6). Thus, the matching contributions must satisfy the vesting requirements of §1.401(k)–1(c) and be subject to the distribution requirements of §1.401(k)–1(d) when they are contributed to the plan.