This has to be one of the most riduclous statments, in adult life are you really going to buy a car, a house or take a loan out because google thinks it's a good idea ? People need to be able to sense check even simple things like knowing you will have enough for the supermarket bill at the checkout. Being able to do this stuff quickly in your head is a massive advantage in life.
I'm bad at mental arithmetic but good at maths - have maths A-level- but still have jumping in points for time tables. Turns out I was dyslexic with poor working memory so mental maths rather than pen and paper is harder.
DH had a maths degree - yet it was me sorting though the mortgages using on-line calculators and best buy lists before running though numbers with him and mortgage company. I do rough guestimates for tills or on-line shop.
It's like knowing my left and right - I don't "know" have to rely tricks or working it out - it's a pain in the arse - but it doesn't stop me living a perfectly normal life.
Our children take after me unfortunately but I did get them fluent in timetable and number bonds - took Percy parker, 4 years of daily mathsfactor and then other on-line sites for more practise - and the school.
I was told DS who just got A* in numeracy and maths and distinction in additional maths - would never be good at maths and was likely had dyscalculia by his first primary school. We were actually told he didn't have a maths brain.
What he had was an even poorer working memory than me glue ear and a school teaching maths though fun group games - what he needed was clear explanations and huge amount of practise/overlearning by actually needing to use the numerical facts in sums and understanding that while he may struggle with mental maths bit he didn't with understanding the concepts - eg for me an example would be having 7 X 8 be a blank spot well into 30s but understanding it was same as 7 *7+7 which could quickly work out.
https://ed.stanford.edu/news/learning-math-without-fear
I found this interesting piece - as it's what I think happened to my DC in Primary because they weren't fast with rote learning maths facts they were written off and if we hadn't step in they have stopped trying - but I think they are like DH flexible with numbers but they encountered more than one teacher who thought there was only their way to an answer.