Good luck with that at a German bank.
The mortgage is likely interest only so the landlord can make approximately £10k/year from rent from that one property. The next year they buy another, their earnings are now £120k. It now only takes them a few months to save another £50k to buy their next property, and the more properties they own the faster it is to acquire the next one. After a few years of this let's say they have 10 properties in total. They are now earning £200k, which makes the bank's decision to lend them more money an easier one, even though they have £1.5m in mortgage debt already.
If they already have 10 properties then it's not that much of a relative leap to get to 20, then 30. With 30 properties they're earning £400k a year and can finance a new property every 2 months. They do have £4.5m in mortgage debt that is not being paid off though, but they're making a bet on the value of those properties exceeding the purchase price by the time the balance of the mortgages becomes due 20 years from now. In the past that was a pretty safe bet.
The risk of doing all of this is that the property market can slow down, rates can go up, values can fall, property maintenance can become a significant cost, bad tenants can ruin a property and wipe out several years earnings and the government can change tax rules that makes all of this far less profitable (as happened in the UK quite recently). But it's absolutely not impossible.
About a decade ago I worked with a perfectly normal guy in his mid 50s who worked as an accountant in a small software company, he had 200 BTL mortgages! He just kept reinvesting the profits and buying more each month.
Purchasing any property is a many-month endeavour with solicitors/banks etc
There are specialized firms w/ private capital that are happy to fund these midsize deal flow guys and finance them. Once you get a working relationship with a few such vendors it seems rather easy to get marginal deals done others would find impossible.
I don't know if this simply doesn't exist in the UK at all - but I'd bet money that it does in some form. Since bankers and finance make their money off the points on deployed capital, especially the loan officer making commission on the deal flow, the incentives are aligned to make more loans vs. less.
I think the hardest part of getting something like this going is the first 2 to 3 properties. After that and a few years of proven ability to manage cashflow here in the US your lending options open up exponentially both in number of lenders and the creative/exotic means of financing now available.
No German bank is going to give you 100% mortgage (in fact 110-120% for taxes/fees/additional reconstruction) when you already have 10 properties mortgaged in exactly same way. You also don't build any cash reserve like that, owning properties is always more costly than financial projections make it so.
At the end it doesn't matter, good for him (apart from everything that makes a man the man) but his vague background story as told simply ain't true, and folks were eating all that till I came along and thought about numbers for few seconds.