That's exactly what the government wants a charity to do not create reserves. It would make sense to spend that money on increasing market share through advertising.
You'll notice that that document has a concept of reserves that are 'too high' - see for example section 4.2 the heading "Where a charity’s reserves appear to be too high
".
I looked up Let's Encrypt on Wikipedia by the way because I didn't remember that being a Mozilla thing - they don't appear to come under the umbrella of the Mozilla Foundation financially and their budget is $3.6 million according to the sidebar, and a literal rounding error if we stick to integer percentage points of Mozilla's revenue.
The UK seems to disagree, for example: https://www.gov.uk/government/publications/charities-and-res...