That study suggests that 65% of autistic individuals are undiagnosed. The 90% is only for middle-aged and older.

Skimming a couple (due to time constraints) of the sources mentioned in the study, one source [1] mention that "3 in 10 autistic people of working age are in employment", and the other [2] that "approximately 50% fail to achieve independence in their living status, employment, and close relationships"

It doesn't seem to support the claim that "90% of autistics are undiagnosed and work".

[1] https://www.gov.uk/government/publications/the-buckland-revi... [2] https://link.springer.com/article/10.1007/s10803-020-04763-2