So the listing as a PHB race is a bit misleading then no? This caused quite a discussion at an AL table recently. I really appreciate the work DnD Beyond does and I do not blame them for oversights (we are all human) but maybe a note in the race selection screen may help things?
You are right about the source and page number. However, the first line of the Variant Aasimar states:
This aasimar variant originally appeared in the Dungeon Master's Guide as an example for creating your own races.
The only reason why the PHB appears as the source is that this variant Aasimar is free. The source is actually Basic Rules for the purpose of making this race appear for free users. I am sure the developers' team will get things cleaner.