I saw that awhile back too ; I think it was a Dr. Campbell presenting, when I saw it.
I have seen other commentary that it doesn’t matter, simply because muscle is very highly perfused, and injecting into the muscle is bound to introduce a lot of microlipid containing the MRNA, directly into the bloodstream anyway.
...of course, it might be that the initial immediate amount into the blood matters too.
Here is a quick link I found, there were others, but it is time for me to go to sleep. Feel free to search further. I don’t think this was as definitive as the material I saw earlier. Do we still trust Lancet?
https://www.thelancet.com/journals/eclinm/article/PIIS2589-5370(21)00054-7/fulltext