Am not sure why she comments that the scenarios “start at 40%” more transmissible, they range from 10 - 50 %.
She comments that SAGE bad scenarios start from 40%.
If B.167.2 is "only" 10-30% more transmissible, the exit wave will be relatively small and manageable from the healthcare perspective.
If the transmission advantage is 40% or higher, the exit wave might grow larger very quickly.
However, there are further assumptions in SAGE modelling that are already out of date. For instance, vaccine protection against hospitalisations appears to be much higher than assumed 90% (extra 5-8% will significantly change the model). On the other hand, B.167.2 has developed some degree of immune escape which is a negative.
James Ward (the mathematician on twitter) runs his own models and I will not be surprised if they end up more close to reality than SAGE estimates. Currently the size of the exit wave based on his calculations is still unknown. We can only wait and see what happens over next couple of weeks.