I have a feeling that they just went with the house purchase because it was available at the moment and other things they looked at were not out on the market- or did not look as good to them. I think that response is more of a *we kind of regret purchasing this house, we are feezing* and she is just saying that in case they sell of the house in a couple of years.
Not to be mean or anything, but Annas salary is in the 300 000 and Mark is probably getting something similar (can't remember right now what his wife is paying him). They don't pay for heating, get clothes for free and have food sponsorships etc. They sold their old flat with profit. If you count all of this, I can't believe they have not been able to buy a house with cash only. Take Tara Michelle for example, she owns a place in LA (US) and has just gotten a fixer-upper in Toronto (Canada) - she could have gotten ready to move in the house but she just did not want to sell her LA place and wanted a low mortgage so the fixer-upper was perfect. Like, I know that influencers have a lot of income but holy tit in a poop stick it is insane!
If they are so cheap as they might appear, they both could have saved most of their earnings for two or three years ad have a mortgage-free house. So, I see no excuse why they would not be able to pay down their mortgage in the next few years if they are expecting the influencer marked to dry out by the end of 2021.