Yeah, and to simplify further, the atmosphere's got a fixed amount of methane-removing capability, and as we exceed this capability it can't absorb methane any

Yeah, and to simplify further, the atmosphere's got a fixed amount of methane-removing capability, and as we exceed this capability it can't absorb methane any faster, so the ratio of methane that gets absorbed starts dropping even though the amount being absorbed doesn't drop. The reality is more complicated because the cut off is very blurry - absorbsion does still increase as methane increases, just not fast enough to keep up, and it falls further behind the more methane we put out.