If you book in advance, usually 30 days or more, yes Heathrow Express is £10.
If you happen to have a National rail card you get a further 33% reduction.
After the 30 day deadline and on the day it's full price, of £25.
If going on by tube to elsewhere in Zone 1 and doing no other trips that day then the added tube ticket still works out cheaper than an Elizabeth Line fare.
But Heathrow Express doesn't count towards your daily or weekly TfL cap.
The essential reason is that the Elizabeth Line has a substantial premium within its price to use the airport tunnel, otherwise it would either be the tube fare from Heathrow or an even cheaper standard zone 6 fare. A mayoral decision to help pay the cost of the Elizabeth Line construction, and generally balance TfL books.
Even a tube fare from Heathrow is higher than it should be due to an airport premium fare.