It's a "bioactive glass" (AKA "bioglass"), like the NovaMin toothpaste that Sensodyne used to sell in the US. Specifically:

https://biomin.co.uk/products/biominr-f-toothpaste

The award winning new formula used in BioMin® F toothpaste slowly releases Calcium, Phosphate and Fluoride ions. These combine to form Fluorapatite in and around exposed dentine tubules to reduce sensitivity and strengthen and protect tooth structure.

https://biomin.co.uk/products/biominr-c-toothpaste

BioMin® C the FLUORIDE FREE daily toothpaste formulated to adhere to the tooth surface and slowly release mineral ions in a similar manner to BioMin® F - only without the fluoride and much faster – to produce a hydroxyapatite coating.

Are they absolutely certain that if accidentally ingested (or ingested in miniscule amounts), that this won't interact with endothelial surfaces or have other undesired affects?

The mechanism scares me. I'd be worried about vascular health, cancer, etc. Probably nothing to worry about, but it's a very novel approach and it's got my mind wondering.

It contains titanium dioxide, for which there are plans to ban it across EU. Though, there isn't anything definitive about it like "you need such and such exposure for so many years to increase your risk of getting cancer by X%".