if you're getting into the weeds, you should provide the full quote so it can be parsed. https://www.toyota.com/t3Portal/document/omms-s/T-MMS-18Highlander/pdf/T-MMS-18Highlander.pdf
The recommendation is only meant to cover the lowest common denomiator of a 5w20 mineral oil. There is not quite as specific an instruction that you can make regarding a 5w20 synthetic oil so on that middle ground you are somewhat on your own to decide.
From Toyota's point of view, they are "certifying" saying they tested TGMO 0w20 for 10k, and also implicitly certifying any 5w20 mineral for 5k.
They didn't test 5w20 so they have nothing to say about it and can't claim it's "certified".
The line items indicate
NOTE: If 0W-20 oil WAS NOT used at
the last oil change, replace engine oil
and oil filter
but the footer appendix goes into the detail
NOTE: Your vehicle is certified with
Genuine Toyota 0W20 motor oil. For
VEHICLES CERTIFIED FOR 0W20 motor
oil, the oil change interval is 10,000 miles
or 12 months IF 0W20 MOTOR OIL IS
BEING USED. If 0W20 is not available,
5W20 mineral oil may be used. However,
it must be replaced at A 5,000 miles or
6 month INTERVAL with 0W20 motor oil.