Bitcoin was created in 2009, ethereum in 2015. Of course, then they did not count on their so high popularity and such a simultaneous number of transactions. Therefore, their scalability was also small. If Bitcoin can still be criticized for this now, then from December 1, the ethereum network is completely updated and one of the planned improvements is to increase its scalability.
Elon Musk is not a specialist in cryptocurrency, and given his contradictory statements about it, I don't think you need to pay attention to his words and, in particular, to his obviously undeserved advertising of Dogecoin.