Well, it's true. Here [1] is the corresponding law, here [2] is wikipedia with some expamples what counts as passive weapons (plastic film, basecaps reinforced with plastic, gasmasks, knee pads, … ) and here [3] is a case where a paramedic was sentenced for wearing a helmet.

[1] https://dejure.org/gesetze/VersG/17a.html

[2] https://de.wikipedia.org/wiki/Schutzwaffe

[3] www.labournet.de/interventionen/grundrechte/grundrechte-all/demonstrationsrecht/schutz-durch-selbstschutz-demonstrationssanitaeter-wurde-wegen-vermummung-und-passivbewaffnung-verurteilt/