Not sure this is correct.
As I understand it, the requirement for ADS-B goes hand-in-hand with that for a mode C transponder. Sort of.
From FAR 91.215(b)
(3) Notwithstanding paragraph (b)(2) of this section, any aircraft which was not originally certificated with an engine-driven electrical system or which has not subsequently been certified with such a system installed, balloon or glider may conduct operations in the airspace within 30 nautical miles of an airport listed in appendix D, section 1 of this part provided such operations are conducted—
Unfortunately the regs read a bit different re ADS-B.
From FAR 91.225(e)
(e) The requirements of paragraph (b) of this section do not apply to any aircraft that was not originally certificated with an electrical system, or that has not subsequently been certified with such a system installed, including balloons and gliders. These aircraft may conduct operations without ADS-B Out in the airspace specified in paragraphs (d)(2) and (d)(4) of this section. Operations authorized by this section must be conducted—
Key difference-- "engine driven electrical system" for transponders vs "electrical system" for ADS-B.
In other words, even a battery-only electrical system triggers the ADS-B requirement.
That's how I read it anyways.
https://www.ecfr.gov/cgi-bin/text-idx?node=14:2.0.1.3.10#se14.2.91_1227