I don't think he's referring to anything specific. "May be" is not a definitive statement, it just seems he is suggesting that it is plausible that a vaccine could be less protective than regular mask wearing. "I might even go so far" couches it even more.
I think that's probably not a great statement to make, either, a better one might be to suggest that "because a vaccine is not effective in everyone that gets it, mask-wearing may continue to be important even with a vaccine", but not every quote in front of the media is ever ideal. Really, the rest of his statement:
if I don't get an immune response, the vaccine is not going to protect me. This face mask will
seems like the part to focus on.
For comparisons to influenza, this page talks about the annual flu vaccine efficiacy:
A quote, emphasis mine:
IIV3 provided statistically significant protection against both the included B lineage (66%; 95%CI 58, 73) and the non-included B lineage (51%; 95%CI 36, 63) during the 2012– 13 season
Tricco, A. C., Chit, A., Soobiah, C., Hallett, D., Meier, G., Chen, M. H., ... & Loeb, M. (2013). Comparing influenza vaccine efficacy against mismatched and matched strains: a systematic review and meta-analysis. BMC medicine, 11(1), 153.
is a meta-analysis of flu vaccine efficacy in children. They write, quote, with emphasis mine:
The live-attenuated influenza vaccine (LAIV) showed significant protection against mismatched (six RCTs, VE 54%, 95% confidence interval (CI) 28% to 71%) and matched (seven RCTs, VE 83%, 95% CI 75% to 88%) influenza strains among children aged 6 to 36 months
VE was calculated using the following formula: (1 - relative risk × 100%)
So it seems that 70% would be in the ballpark of the annual flu vaccine against the included strain. Obviously the 2019 coronavirus is not influenza, but we have no solid long-term efficacy data yet, and I think it's reasonable to expect that even a very useful vaccine (on par with the annual flu vaccine given to millions) would not approach 100% efficacy. Even with a vaccine, it will probably be important to continue basic public health protocols intended to limit virus spread, especially before many people are immunized.
There's some mixing here between "immune response" and "efficiacy" but I think it would be fair to say that if someone gets sick, then whatever measured immune response they may have had was not effective, so we might as well consider it not an immune response for this purpose. Rewriting a paraphrased version of the quote, I would say:
"If the Covid vaccine provides similar protection to the annual influenza vaccine, the immunogenicity could be around 70%. If I don't get an immune response, the vaccine is not going to protect me but a face mask will still provide some protection."