Short version - this is unscientific bollocks.
For Biologists, sex is binary; intersex people are either male or female with unusual development patterns.
To claim sex is a spectrum, you have to define what the X axis is measuring.
To see the arguments in action have a look at:
https://twitter.com/lecanardnoir/status/1503857738148892673?cxt=HHwWgsCysaSp494pAAAA
or
https://sex-matters.org/posts/updates/doggit/
Aber has just saved me a fair bit of typing, thank you.
Sex is about the most binary thing there is. There are only two of them, everybody has to be one or the other. There is no third sex.
People with so-called "intersex" conditions are male or female like the rest of us, just with anomalies in how that is expressed. Someone with an abnormally formed genital tract, male or female, still has a genital tract of the appropriate sex, just as people with a cleft palate still have a mouth. People with disorders of sexual development generally want nothing more than to be accepted and regarded as the sex they are (again the parallel with a cleft palate is relevant), and bitterly resent people co-opting them into a dispute that isn't theirs and implying that they are in some way not "real" men or women.
It is true that some of these disorders cause mistakes to be made when sexing newborns, but these are seldom persistant in developed countries where proper medical investigation will soon reveal the child's actual sex. The number of births where it is genuinely difficult to figure out which sex the child is, and detailed chromosome and hormonal analysis has to be undertaken, is vanishingly small.
Caster Semenya is a good case in point. Apparently he has 5-apha reductase deficiency, which is a disorder of males and causes ambiguous external genitalia which appear female in the newborn. In most cases more-or-less normal male genitalia appear at puberty, although I gather the male genitalia can remain internal in a small number of cases. Everyone close to Caster Semenya must have known he was a boy since he was at least in his mid teens. Serious deception going on there. (I gather he now has two children by his "lesbian" wife.)
To accommodate anomalies more complicated than that one, the following definition of sex is useful.
Male: has both a functional SRY gene and functioning androgen receptors.
Female: has (typically) no functional SRY gene, or (rarely) no functioning androgen receptors.
Even mosaic conditions are covered by this, since someone who has an apparently normal SRY gene in one set of body tissues still isn't male unless that gene is
expressed in such a way as to have steered the body down the male development pathway.
But most of all, this has ABSOLUTELY BUGGER-ALL to do with the trans issue. People with intersex-type disorders are very very rarely trans, unless they are actually transitioning to rectify a mistake made when they were infants (like Erik Schinegger, who appears to have been another Caster Semenya). To a very good estimate, 100% of trans people are genotypycally and phenotypically normal members of the sex they were born as, but present with a mental condition.
Whether the best way to treat a mental condition is with hormones and surgery, and whether it is reasonable to demand that the person should then have all the social and legal rights pertaining to the sex they aren't, is a whole different discussion. The discussion we're actually having, in fact.