The body has a mechanism to move Na+ across the intestine wall. If it didn't we'd be in trouble. There's a description here: http://www.vivo.colostate.edu/hbooks/pathphys/digestion/smallgut/absorb_water.html#:~:text=Sodium%20is%20absorbed%20from%20the,the%20lumen%20into%20the%20enterocyte. Yes, it is from the small intestine section, but the process is similar in the large intestine.
Absorption of normal saline is slower than absorbtion of tap water, but it still happens. That's the reason that osmotic laxatives aren't made with chloride salts, but rather PEG or sulphates or phosphates. Indeed, hypertonic sodium bicarbonate or chloride enemas often have notes attached to watch patients because if they don't expel there is a risk of hypernatremia (dangerously high blood sodium) - there is one case study of a man who died from a Mayo enema that was retained. Mayo enemas are very hypertonic with sodium bicarbonate as well as sugar.
I'd trust Eric as he actually has medical training more than this, but what I have seen seems to indicate that while large quantities of tap water may have a theoretical risk, in most cases with healthy people and reasonable amounts your body will work around it. A day or more consisting entirely of enemas might not be a good idea, but a couple fills seem to not be an issue unless you have renal problems.
The other issue may be inflammation of the intestinal wall. In single-ingredient enema studies (one US, one Japan) the use of normal saline had extremely low or nonexistent inflammation of the rectal wall. Tap water had more, soapsuds next step up, and most inflammation was hypertonic glycerin or phosphate enemas. Mixing soap and saline seems like it may increase the inflammation over straight saline, might still be a bit lower than SSE without saline, but likely not enough to make too much of a difference. Again, I have never found any discussion of what the inflammation "means" - it is just noted as being present.
Both studies noted that tap water and soapsuds enemas were more effective than the saline ones.