This is just my musings on the matter, para 77 is very long winded and goes all around the houses. He does seem to be fudging it a bit.
The judge says that he is "less certain" than the EHRC (who used the word "must") about this point but doesn't say that they are necessarily wrong.
Later in the paragraph he says
"Even though the EHRC’s obligation ... is to provide an accurate statement of the law, the court must apply this requirement recognising that any statement of law will rest on some assumption of fact, even if only generic."
To me, he appears to be saying that the EHRC must have had some specific real-life situation in mind, even if unspoken and/or very generic. Some undeclared assumptions that have been made.
And that it is ok for the EHRC to do that:
"Where a body such as the EHRC has issued guidance that rests on factual premises that are permissible, the court should hesitate before concluding that the guidance as issued was unlawful."
.
The Judge then further went on in para 78. GLP had argued generally that the Update didn't recognise that applying many of the provisions of the EA2010 would be very fact specific and case specific - that there was no room for absolutes, like the word "must".
The judge disagreed:
[78] "... I do not agree. That point was sufficiently recognised at paragraph [2] of the Interim Update. That refers to the “proportionate means of achieving a legitimate aim” test. That is the requirement for factual evaluation that permeates through all the provisions on single-sex provision and the possibility of derogation from the prohibition against discrimination at section 29 of the EA 2010"
So, he seems to be saying (although I may have misunderstood this) that at the initial stage of determining that single-sex toilets were a proportionate means of a achieving a legitimate aim, whatever factual evaluation that would have been done at the time would be the "factual premises" that are assumed for the purposes of para 3a.
Or I may just be hopelessly wrong about everything, I don't know! I did also find para 77 very confusing.