Good question.  This is a statement on the variability (standard deviation) in Nino-3.4 SST, which is shown in Figure 2 above.  The IPCC section 4.5.3.2 says:

"There is no consensus on changes in amplitude of ENSO SST variability across CMIP iterations." 

and

"While CMIP6 models show no robust change in ENSO SST amplitude in the mid- and long-term period across all four SSPs, a robust increase in ENSO rainfall amplitude is found particularly in SSP2-4.5, SSP3- 7.0, and SSP5-8.5"

Also, in Section 4.4.3.2:

"... detection of robust near-term changes of ENSO SST variability in response to anthropogenic forcing is difficult to achieve due to pronounced unforced low-frequency modulations of ENSO."   

and

"Analysis of CMIP6 models shows a slight increasing trend in amplitude of rainfall variability over Niño3.4 region in the near term attributable to mean moisture increase, regardless of changes in ENSO SST."