Even the Mahayana has two basic positions (and variants thereof).
1. The first is that arhat and pratyekabuddha realization is correct, and they have ceased samsaric becoming. Their realization of nirvana / cessation of defilements is the same as that of the Buddhas. However, the Buddhas also have other forms of knowledge, overcoming obstructions to the knowable. However, these obstructions to the knowable, which the two vehicles have, are not obstructions to simple liberation.
2. The second is that the realization of arhats and pratyekabuddhas is actually incorrect, and not actually liberation at all. Rather, they enter some sort of stasis period, from which they later re-arise, and find out that they are still in samsara. They then turn towards the Mahayana path, which is the only path to liberation.
Of these two explanations, it appears that the former is the earlier. There is debate amongst the holders of each, as to which is "explicit" and which the "implicit / provisional" teaching.
For the former, for a bodhisattva to attain arhat / pratyekabuddha awakening is disaster, because they are finished in samsara. Game over. However, for the second system, it is a big delay, indeed, but not the end, because all of the two vehicles have to come back to the Mahayana anyway.
In the Far East, with the dominance of the Lotus Sutra, the second view is sometimes very common. However, even there, teachers with experience in India, like Xuanzang, still upheld the former. Most of the systems based on Indic texts follow the former system, eg. the Abhisamayalamkara, and Sanlun, Faxiang, etc.
It seems that our friend Dexing is expressing a very common position from East Asia, which has it's own tradition and scriptural basis. I merely wish to point out that it is not the only position. And, if we mention a specific school (as above, the "Theravada"), we must consult that school itself, and see what they say, rather than taking other schools' ideas about them. Especially when East Asian buddhism had very little doctrinal contact with the Theravada, for instance.