This is where the scholar and the subject matter become confounded. The fact that you didn't know about this tradition doesn't mean that the early Christians didn't know about it. First of all, it is almost explicit in the text of Exodus. Second of all, the number of Samaritans in early Christianity - Simon, his first associates, Justin etc make Samaritan undercurrents hard to deny. It couldn't be that Simon and Simon (Magus and Cephas) were interacting at such an early date speaking two different languages. There must have been a common understanding of what an apostle is. Yes to be certain the idea of 'apostles' appears to be early because of our canon. But again the Marcionite use of 'apostolic' (i.e. pertaining to one apostle) likely goes back to Samaritan usage (via Simon and the rest) and the Clementines make it seem that Peter used the same logic to deny Simon/Paul the title of apostle (i.e. there can only be one).Therefore, holy brothers and sisters, who share in the heavenly calling, fix your thoughts on Jesus, whom we acknowledge as our apostle and high priest. 2 He was faithful to the one who appointed him, just as Moses was faithful in all God’s house. 3 Jesus has been found worthy of greater honor than Moses, just as the builder of a house has greater honor than the house itself.
Only in the Gentile world did 'apostolos' take on a generic meaning. The Hebrew makes clear that Moses is the sent for one. God sends him over and over to speak on his behalf. The role of Paul is similar if we image the gospel being introduced by Paul (as the Marcionites held). Clement for one sees Paul emerge immediately after the resurrection. Paul himself says he ascended to heaven (really ascended not in his mind) and saw and heard Moses and clearly received 'secret' instructions there. For the Clementines this sort of a revelation isn't Mosaic enough. This, according to the author, is more like a prophet than an apostle. Moses stood in front of the burning bush and was 'sent.' So was Peter (notice the implicit context is Peter alone is the apostle even though there is a reference to 'apostles' later which I think is secondary). It doesn't make sense to cite the Miriam and Aaron leprosy incident if you mean 'apostles.' The implication is clearly only Moses is an apostle.