Isn’t that fairly common for the first round of human testing for novel treatments like this? I could certainly understand wanting some small scale reassurance that a wider study won’t be throwing good money after bad.
And, as in study notes, to prioritize understanding safety with this population before worrying about efficacy.
It is quite normal to start with small groups for economic reasons. If the first results look promising, you can scale up. If they are a wash, you haven't spent too much money on chasing yet another placebo.
Also an opportunity to further design or evolve the study.
Not just economic (although it definitely ties into it) -- finding an eligible patient population also another complicating dimension.
The problem is that invasive novel treatments like this always have a large placebo effect, so not doing a placebo controlled trial is essentially a waste of time and money.