It's just a "6+" on the Japan Meteorological Agency seismic intensity scale, which does otherwise fit the description about being 1 less than the maximum on that scale, but the "M 6.9" actually refers to the value in the more universal moment magnitude scale (and just happens to also start with 6 in this case). When intensity is mentioned it's the 0-7 scale, when M or magnitude is mentioned it's about the moment magnitude scale.

https://earthquake.usgs.gov/earthquakes/eventpage/us6000t7zq...

You're conflating two things, magnitude and intensity.

This earthquake was measured as a 6.9 moment magnitude, sometimes referred to incorrectly as the "Richter" scale.

The Shindo intensity system is measured without decimal places. In this particular case, the intensity was a 6+. Which isn't the highest, but is still quite severe.