I would suggest that the page is actually an error.
If you read the Countries that allow for Transit Without Visa it includes: Afganistan, Iran, Kenya and so one but doesn't include any of the countries of the European Union, USA, Japan, Australia, New Zealand or any other countries of Commonwealth, which is at the very least odd.
I would suggest if you don't believe the questionnaire that @NateEldredge suggested you can use Star Alliance website to access Timatic which airlines use to determine whether or not you need a visa.
Generally speaking Kenyan citizens need a visa for Transit, however, if the visa for Canada is valid then they can transit through London without Visa if she stays airside and:
travelling as part of a journey FROM either Australia, Canada, New Zealand or the USA, provided transiting the UK less than 6 months after the date they last entered Australia, Canada, New Zealand or the USA with a valid visa for the respective country, even though the visa may have expired at time of transit through the UK (e.g.: JFK-LHR-DEL, JFK-CDG-LHR-DEL or JFK-LHR-CDG-DEL).
So if she meets the criteria they should have allowed her to board.